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Figure 2.1:

A combination of Stellarium with the Unity game engine

This combination can provide a lively and attractive simulation environment which allows operating virtual reconstructions of historical astronomical instruments under a temporally and culturally matching sky. This “serious game” presents instruments constructed and used on Ghāzān Khān’s command in Marāgha, ca. AD 1300 (Mozaffari & Zotti, 2012). The sky is provided by Stellarium using an inter-program link and in this scene shows the sky with constellation artwork from a manuscript of aṣ-Ṣūfī’s (903–986) Book of Fixed Stars.

Exploring Skies Remote in Time and Culture with Stellarium

Georg Zotti (Vienna, Austria)

Abstract

The free and open-source desktop planetarium “Stellarium” provides a multitude of capabilities for research and demonstration in astronomy. This paper presents application highlights in the fields of virtual archaeoastronomy which was one driving force of development, sky culture research with a layout of possible future developments, and some notes of caution with pitfalls often seen in practice.

Zusammenfassung

Das freie quelloffene Planetariumsprogamm “Stellarium” kann vielfältig für Forschungs- und Simulationsaufgaben genutzt werden. Dieser Beitrag stellt es im Forschungsfeld der “Virtuellen Archäoastronomie” vor, in dem Stellarium mittlerweile eine führende Rolle spielt. Auch für das nichtmaterielle Kulturerbe der Himmelsmythologie bietet Stellarium mit einer Vielzahl austauschbarer Sternbildkulturen viefältige Anwendungsmöglichkeiten, die künftig aber noch deutlich erweitert werden sollten, wozu einige Ansätze erwähnt werden. Einige weitere Hinweise um die Handhabung des Kalenders, zur Simulation von Finsternissen, sowie zur Verknüpfung mit anderen Programmen ergänzen die Darstellung.

2.1 Introduction

The sky has fascinated and interested humans since their earliest days. Archaeological finds of incised bones from Palaeolithic times have been interpreted as Lunar calendar tally bones (Rappenglück, 2015a). Also since these early times humans have seen animal figures formed by the fixed stars. The cave paintings of Lascaux contain depictions of the Pleiades star cluster above an aurochs bull in the same figure known today as Taurus (Rappenglück, 2015b), and figures of the Celestial Bear can be found in most European and northern Asian and even American cultures, hinting at an archaic origin (Frank, 2015a, 2016; Bäcker, 2020). Early Chinese oracle bones likewise contain inscriptions about celestial phenomena, in the more general sense of both astronomical and meteorological observations (Pankenier, 2015). Many of the constellations which we use today can be traced back far into antiquity or even prehistory (Frank, 2015b). The best-known collection of constellations from antiquity certainly is included in Ptolemy’s Almagest (2^nd century AD). After centuries of discoveries and ever more extensive celestial cartography which had to fill the hitherto uncharted southern sky with new constellations, the sky has been finally partitioned into 88 regions for a canonized collection of constellations (Delporte, 1930) which in the modern “Western” scientific use and understanding of astronomy are merely used to address stars. The study of skylore of other peoples is a topic usually performed by ethnologists, but popular in astronomical outreach activities.

Apart from this immaterial astronomical heritage in form of starlore, human cultures worldwide have left traces of profane and sacred buildings or monuments. Frequently the orientation of architectural axes and sightlines can be linked to directions provided by nature in form of the rising or setting positions of the Sun at annual key dates like the solstices, extreme declinations of the Moon, or even of bright stars. Examples include megalithic stone circles of Great Britain (Ruggles, 1999), the Mediterranean (Hoskin, 2001), Egypt (Belmonte & Shaltout, 2009), Greek temples (Ranieri, 2015) or sites in pre-Columbian America (Šprajc, 2013; Ziółkowski et al., 2013). Countless more examples of what is now contained under the term of “cultural astronomy” have been presented over the last few decades (Ruggles, 2015).

Unfortunately, the scientific divide of research disciplines has led to archaeologists who are often unaware of celestial phenomena and their widespread cultural impact, while today’s astrophysicists rarely deal with historical observation techniques or sacred architecture. Even if there may be contact between researchers on particular questions, a first difficulty may lie in finding a common language. While astronomers and astrophysicists must learn a bit of history and (depending on the research topic) may have to tip their toes into social sciences or the humanities, archaeologists dealing with archaeoastronomical topics should understand basic astronomy on the level usually found among amateur astronomers.

Such interdisciplinary work can be supported by modern tools. The classical tool for analysis of the sky visible at a particular date and time was the planispheric star map, which used a horizon mask and circular date and time scales to reveal which part of the sky was visible. Nowadays, desktop computer programs have almost replaced these tools, and they can provide many more possibilities than the mechanical devices ever were able to present. One of these tools is the free and open-source desktop planetarium “Stellarium”. It has been developed by (mostly) amateur and professional astronomers, some of them with strong background in computer science and computer graphics, and is based on many scientific models from astronomical phenomenology and human psychophysics (Zotti & Wolf, 2020a; Zotti et al., 2020b).

2.2 Virtual Archaeoastronomy

Archaeologists nowadays are using modern survey instruments like total stations to record their excavations. These data are imported into CAD (computer-aided design) or GIS (geographic information system) projects to create detailed maps. Outlines of foundation walls can be drawn, and together with 3D artists and architects, virtual reconstructions of decayed buildings and monuments can be created. Tools from the 3D architecture and the computer gaming industry can be used to recreate whole settlements in their landscape, place virtual users into this scenery and let them explore and even interact in such virtual environments just like in the popular first-person perspective computer games. Such recreations in “virtual archaeology” should adhere to certain standards to document which parts of the reconstruction are built on solid data and where artistic and interpretative liberties set in (Denard, 2009; Seville, 2011). A proper, archaeologically sound graphical simulation of a landscape needs good collaboration and understanding between archaeologists and computer graphics experts.

Unfortunately most architectural rendering systems or game engines present their scenes under a daylit sky without too much attention to the accurate placement of the Sun or, even less, the other celestial luminaries. While the position of the Sun can be computed with moderate effort, a complete, historically accurate and realistic sky simulation module for a game engine has not been available until recently.

A computer program applicable for virtual archaeoastronomy and historical astronomy needs input from several disciplines (Fig. 2.2):

• Astronomical Phenomenology. This encompasses most of “classical astronomy”, like observation techniques from the pre-telescopic era and a good understanding of astronomical phenomena visible in the sky.

• Archaeology, history and related fields. Obviously, when researching archaeologically determined remains of past cultures and their possible links to celestial observations, these have to be analysed in context with the knowledge gained by archaeology and historical research. The sky simulation must include some representation of the archaeological site in question. Since the 1990s, computer desktop planetarium programs have been capable of adding a decorated horizon. At first this was just a polygonal mask which can represent the general shape of hills and mountains. Later programs can decorate the horizon with a photographic panorama which, when properly adjusted, can provide a reliable proxy for the actual landscape horizon visible from one particular location.

Figure 2.2:

The multidisciplinary nature of Virtual Archaeoastronomy (VAA)

• Computer Graphics. Good visualisation is important for demonstrations and communication of research results to a wider audience, but can also greatly help the astronomical non-expert from other disciplines (historians, archaeologists) to understand the celestial processes.

2.2.1 Why simulation is necessary

There is still no complete simulation environment available which could provide the fully immersive look, feel, sound and other impressions gained when standing outdoors under a pristine cloudless sky. Computer graphics and interactive systems can provide a view window (on the monitor), or at best a virtual reality environment (with motiontracking headsets so that gaze direction follows head movements, and where images are displayed on a screen in front of the users’ eyes). Still, only graphic simulation can provide us with the possibilities to replay observations of possible intended orientation of features in prehistoric monuments, for a variety of reasons:

Some archaeological sites are fragile, and extended stays may endanger the integrity of the site. In other cases they lie in remote countries, so that travelling, extended stays or recurrent visits become too costly.

Even if we can stay on a well-preserved monument site for a long time, we cannot observe even the simplest event like a solstice sunrise in the exact same way as it could have been observed a few thousand years ago. Earth’s axis tilt is changing, so that the angle on the horizon between summer and winter solstice sunrise points is shrinking. For example, summer solstice sunrise at Stonehenge in the years around 2000 BC was one degree, or two solar diameters, further north, or to the left, of where it appears today.

In addition, Earth’s axis undergoes a precessional motion which causes an apparent shift of the imagined “sphere of fixed stars” parallel to the ecliptic. This causes a drift of rising and setting points of stars towards which some monuments may have been oriented. For example, the stars of the Centaur and the Southern Cross were visible from the site of Stonehenge in the earliest known times of its use (around 3000 BC).

A handful of bright stars have also noticeably moved since begin of recorded history by their proper motion.

An unfortunate civilisatory side effect of about the past century is the increased level of artificial light at night which causes light pollution and has changed the appearance of the sky next to metropolitan areas, where many monuments are located, so that even the quality of the night sky, and thus the experience of the natural sky above such a monument, has changed significantly.

Of course, simulation also allows to speed up practical research: an observing cycle of particular phenomena may take many years to complete. The Moon’s nodal movement takes about 18.6 years to complete, and Saturn’s orbital movement almost 30 years. A simulation can run through these periods and present the relevant views within just a few minutes.

2.3 Stellarium

The development of Stellarium, a multi-platform desktop planetarium program aimed at a highly realistic simulation of the night sky (Stellarium, 2021), was started by Fabien Chéreau in the summer of 2000. When, in late 2008, the author investigated available desktop planetaria for sky simulation in the 5^th millennium BC for a research project in archaeoastronomy (Zotti & Neubauer, 2015), one commercial product with a “pretty” sky simulation showed erratic behaviour around its implementation of atmospheric refraction. The other “pretty” sky simulation was found to be Stellarium, which had the added benefit of being available free of charge. At that time it was targeted at amateur astronomers’ observing requirements, not at historical research, and had noticeable deficiencies in accuracy when compared to the venerable DOS program “UraniaStar” which had been developed also as tool for historical research but was graphically hopelessly outdated (Mucke et al., 1992; Vollmann & Pietschnig, 1995). However, Stellarium’s nature as open-source project promised to allow changes in the code to improve accuracy, add refraction and extinction and many other details that would come over the next years. Extended presentations of the program and its history have been given recently elsewhere (Zotti et al., 2020b; Zotti, 2021), so we can limit the presentation of particular features here to some highlights.

2.3.1 Stellarium for Virtual Archaeoastronomy

Aside from improvements in the computation accuracy of the astronomical engine, the most significant functionality development for aspects of virtual archaeoastronomy was the introduction of a 3D foreground rendering plugin (optional module) which can be used to display and explore, in first-perspective view, an architectural 3D model of any historical monument, temple or other building as long as the model has been properly georeferenced and oriented (Zotti, 2016; Zotti et al., 2019). A virtual model of the “Sterngarten Wien” (“Vienna Star Garden”) (Mucke, 2002) which has been used to develop some of the phenomenological features in Stellarium is included in the regular distribution and can be explored by everybody.

However, an imagined free collection of contributed accurate 3D sceneries of historical monuments relevant to or frequently discussed in archaeoastronomy, e. g. the Pyramids of Egypt, Stonehenge, various temples of Antiquity, Angkor Wat, the Jantar Mantars of India, temples of Mesoamerica etc., could not be created so far: for most sites, no accurate data are available to the author, and if they are, authorities seem to generally not encourage republication attempts of their data and thus popularization of the archaeoastronomical relevance of their sites. The Stellarium team is more than willing to host contributed 3D models of sites relevant to astronomy when released under a creative commons license, especially if there is a clear link between architecture and celestial phenomena.

Sometimes a static architecture and landscape model is not enough. For example, if the use and handling of historical astronomical instruments should be demonstrated in a virtual environment, we need to interact with the objects in the scene. “Game Engines” provide the necessary building blocks for the creation of lively sceneries with nature-like behaviour of wind-shaken vegetation, curly waves on water bodies reflecting the scenes and sky, and objects showing inertia against attempts to push them but also react to gravity. Such an environment is highly attractive for interactive and outreach installations which aim to present historical sceneries, however the sky and any astronomically correct presentation of it have not been the focus of game engine development so far. A software bridge (Fig. 2.1) between Stellarium and the Unity game engine meanwhile allows using Stellarium’s sky rendition as live background in a Unity-based virtual environment (Zotti et al., 2020a).

2.3.2 Skycultures

A popular feature in Stellarium is the display of the constellations in various formats (Fig. 2.3). The classical elements were

IAU borders the prosaic official borders of celestial regions canonized by the International Astronomical Union (Delporte, 1930).

Constellation lines line art which connects the bright stars in a way that it recreates the shape of the region’s namesake. The stick figures are not standardized, and several sets have been developed which can be switched in the program menu.

Constellation artwork a figurative drawing inspired by historical atlases.

Over the years, several shortcomings of the original implementation have surfaced, and the Stellarium team has started to extend this part of the program and will continue to do so in the not too far future (Zotti & Wolf, in press; Zotti, 2020).

In addition to the constellation’s line art, “asterisms” in the modern sense, i. e., stick figures for non-official constellation-like shapes in the sky, like the “big dipper” or “summer triangle” can be shown, and “ray helpers”, long alignements of stars which help orienting in the sky, can be displayed as separate category. A large number of asterisms, traditional ones like the “dippers”, or newer, often telescopic ones (e. g. the “Coathanger”) developed by the amateur astronomy community and popularized in various magazines and observing guide books in the last decades, has been collected over the past few years. But just like the uncontrolled growth of constellations had to be stopped in the 19^th century, the development team sees a need for upcoming changes: it may be required to add further selection criteria to not overload the sky display with ever more overlapping or intersecting asterisms (e. g., the “Winter Hexagon”, “Heavenly G” and “Winter Triangle” in Figure 2.3 formed from the same stars), some of which may be traced to particularly inventive individuals. For example users could be allowed to select only asterisms from particular sources, and/or separate telescopic asterisms from those visible to the unaided eye, to avoid too much overlap.

A similar growth in inofficial but popular names for non-stellar objects could also be observed. Various authors of “deep-sky observing handbooks” and websites which show off the latest images of well-known deep-sky objects (DSO; nebulae, galaxies, etc.), or those which came into reach of amateur instruments only lately, tirelessly coin new names even for well-known objects, or propose a new name for an interesting new target. Some of these names may appear too fanciful, or new names for well-known objects could indeed be considered simply useless to some users. The Stellarium team has started several years ago to add such name entries with a source reference. It may be used in the future to select those references according to personal preference, so that unwanted names from particular sources can be suppressed.

Figure 2.3:

Constellation of Orion and its environment displayed in Stellarium.

The program allows the selection and highlighting of single constellations which can be displayed with a mix of IAU borders, line art and artistic rendering. In addition, “asterisms” (inofficial figures) are shown in green, and long “ray helpers” in yellow.

Also star names are, as it seems, a never-ending popular topic. Most star names traditionally used have their origin in Ptolemy’s Syntaxis where they often simply denoted the star’s position in the constellation (“the one at the hand” etc.). As is well known, this most popular astronomical work of antiquity was translated into the Arab “Almagest” and centuries later translated to Latin, and many “Arab” star names, some of which were added from local traditions, were corrupted into seemingly untranslatable proper names which often only could be explained by modern research (e. g. Kunitzsch, 1959, 1961).

Just like the constellation figures, star names were not originally canonized by the IAU but over the last century had been a topic most actively researched by historians of science and ethnologists. In recent years the IAU has formed a naming body which assigns unique names to stars (Mamajek et al., 2016a,b). However, instead of rectifying former and well-known errors, the goals of this IAU working group are to diversify the global set of names with regard to their origins and to create memorials for nowadays obsolete celestial names (from all cultures). Moreover, “In some cases, ancient asterism names may provide a potential reservoir of new names for individual stars” (Mamajek et al., 2021). This way, some naming criteria, while removing a handful of ambiguous names and spelling varieties from widely available classical sources, unfortunately lead to added confusion and historically questionable results. A few examples: One recommendation is that names consisting of more than one word should be avoided. Therefore the classic Deneb al-Okab (the tail of the eagle) was shortened to a factually defaced name Okab (eagle). Such a name would fit as main star of a constellation only, and not for a secondary one. This is misleading because the name “eagle” had been applied pars-pro-toto to the brightest star of the constellation since antiquity, and the brightest star of the eagle is of course still α Aql, Altair. Likewise, Suhayl (without additions) used to be the exclusive Arab name for α Car, (Canopus), and not, as now “approved” as Suhail, for λ Vel, for which a former name of suhayl al-wazn can be found from aṣ-Ṣūfì (Kunitzsch, 1959, no. 185, n. 2). Giuseppe Piazzi’s (1746–1826) known misapplication or transfer of the Arab name algomeisa for a CMi (Procyon) was “approved”, assigning in historical error the name Gomeisa to β CMi (recte mirzam as common designation of a star that accompanies a brighter one, s. Kunitzsch (1959, p. 117 n. 3, no. 93, no. 143)). The correct but ambiguous name Mirzam was now assigned to just one of the stars formerly bearing this “companion” designation, β CMa. Finally, in what appears like a half-hearted move to “preserve intangible astronomical heritage (cultural celestial names) for modern and future use by the international astronomical community” (Mamajek et al., 2018), several traditional star names originating outside of the Ptolemaic sky tradition (i.e., outside of what is commonly termed “western science”) have found their way into the official list of named bright stars. However, in my personal view, as the names do in no way fit into the context of the “western” constellations, they rather appear like ethnological loot, deprived of their context, as if the sky was a colonial museum or cabinet of curiosities. This hopefully was not the intention.

Just like for DSO, and predating these recent developments, Stellarium has started to trace the origins of its own extensive collection of star names and should, in future versions, allow the user to select which sources star names should be allowed to come from in its default “Western” skyculture: the “official” IAU star list (and to distinguish further, with or without the arcane names for exoplanet systems now also assigned by the IAU after public naming campaigns), selected 20^th/21^st century textbooks, or for example also star lists from GOTO mounts, which can help in setting up telescopes.

One unique aspect of this software not found elsewhere is in fact the possibility to exchange the representation of the 88 officially canonized constellations by other “skycultures”. Stellarium’s own constellation artwork was created by the Belgian artist Johan Meuris. It may be attractive to create accurate representations of the classical atlases by his four namesakes:

• the Uranometria (1603) by Johann Bayer (1572–1625),

• the Firmamentum Sobiescianum (1690) of Johannes Hevelius (1611–1687),

• John Flamsteed’s (1646–1719) Atlas coelestis (1729) or

• the Uranographia (1801) of Johann Elert Bode (1747–1826).

Another attractive candidate to be presented in the program would be the Coelum Stellatum Christianum (1627) of Julius Schiller (1581–1627) who proposed to replace the “heathen” constellations by figures from the Old and New Testament.

This would only require high quality scans and enough time to digitally clean the atlas scans from gridlines and stains, isolate the constellation drawings and possibly re-assemble fragments from adjacent maps, and optimally re-project the figures to avoid distortion when anchoring them to Stellarium’s sky with just 3 stars. It may however also require development of more accurate anchoring possibilities.

Stellarium (as of version 0.20.4) comes with over 40 skycultures, although of varying range of completeness and quality. Some of them are variants of the “Western” skyculture, just with different constellation line art or artistic renderings. The majority are presentations of non-Western constellations provided by the community of users from around the globe, which opens Stellarium’s applicability for ethnoastronomical research, education and outreach by creating ethnographically contextualized sky views, with constellations and star names as they were and are still used in other parts of the world, often by cultural minorities or small indigenous peoples. In this respect also Stellarium’s general translatability into more than 80 languages needs to be mentioned, which allows preservation, propagation and dissemination of the intangible cultural heritage of starlore on a very low threshold. Sometimes such a contribution is accompanied by a localized horizon foreground which is then made available as optional download from Stellarium’s online collection. Several rich and beautiful contributions have been created and donated to the Stellarium project from various parts of the globe or also for historical cultures (e. g. Hoffmann, 2020), however without any real possibility for the developers to assess accuracy or completeness of the presentations. Currently, if a contribution looks formally correct, reasonably complete, interesting and serious, it will usually be accepted into the distribution. Recently, a classification scheme was introduced to discern historical from ethnographical descriptions, or from those which come from representants of some cultural group who describe their own living (and maybe endangered) culture. To express a personal opinion again, it appears to me as the by far more complete approach to preserve this part of immaterial cultural heritage, names of single stars and constellations, celestial mythology and constellation knowledge, in complete form with as much contextual description as possible and needed, instead of just picking one or two star names per “underrepresented” culture and engrafting these into the classical European canon of constellations.

Figure 2.4:

A few skycultures from Stellarium 0.20.4. From top: (a) default “Western”, showing IAU borders and modern artistic drawings. (b) Variant after Rey (1952), drawing the figures by “line art” exclusively. (c) Romanian.

Figure 2.5:

A few skycultures from Stellarium 0.20.4. From top: (a) Babylonian (Hoffmann, 2020) (b) Egyptian (c) Chinese including irregularly partitioned lunar mansions.

Over the past years, though, some further shortcomings of the current implementation have surfaced. A few cultural concepts have to be studied better before a thorough implementation can bring also those ideas into the program (Zotti & Wolf, in press). Some further ideas for possible future development should be mentioned here, without expressing a commitment that they will appear in the nearest future.

Currently, all constellations are treated equal, however this does not reflect common perception. There are the classical 48 constellations from the Almagest which are generally better known to at least most amateur astronomers than some more elusive ones introduced much later. It may be useful to highlight or group in some way, maybe by color-coding, constellations which belong together, maybe because they had been introduced by a particular author, or because they are linked by a common mythological story, like the legend around Cassiopeia, Cepheus, Andromeda, Cetus, Perseus with Medusa’s head, and Pegasus. And for common people in the Western world, the constellations belonging to the 12 Zodiacal signs are the best known of all. It appears to be useful to allow marking selected groups of constellations maybe even by various and non-exclusive or overlapping criteria. These criteria may however become so diverse that only the technology may be provided, but the actual lists will have to be assembled by the users according to their own criteria.

Another possible extension would be the introduction of temporal aspects, i.e., showing constellations only after they had been introduced by particular authors. All these additions are however again mostly useful for the long and evolving tradition of the “Western”, and possibly the Chinese skyculture, the history of which can be reconstructed quite well at least since Ptolemy (2^nd century) or the Suzhou map, respectively. Most of the other skycultures usually show snapshots created by single authors, where only few of these desires may be important.

In many Asian cultures, the Lunar Stations (or Lunar Mansions) in the form of 27 or 28 areas along the ecliptic (see an attempt to represent the Chinese Lunar Mansions using technically “constellation borders” in Fig. 2.5(c)), or of concrete asterisms (in the more general sense of “starry stick figures”), are in a similar range of importance as the Zodiacal constellations (or just the “zodiacal signs” as 30-degree sections of the ecliptic) have been in the cultural “West”. Currently some contributions show them marked as “constellations” or “asterisms” if they can be shown as “stick figures”, or even, by repurposing the “constellation borders” as geometrical regions in the sky, but these should indeed be highlighted as particular group. Traditional calendars and festivals may be linked to them, and they also play a role in the respective astrologies, like the Zodiacal signs are related to European astrologies. But here is also a bit of a catch for Stellarium or other astronomy programs: modern science rightfully despises astrology as humbug. How much of cultural astronomy elements which can be useful to understand previous or culturally diverse concepts should a generally useful astronomy program show before users not inclined to cultural astronomy issues will begin to ask questions? Implementation of primarily “astrological” features (which can be justified as long as they would be limited to astronomical and geometrical features required in historical/cultural context) may be allowed in future development, but only as optional plugins.

Another, less disputable addition should be that of “negative” or “dark constellations” formed by interstellar dust clouds in the Milky Way. These are best kown from Australian Aboriginals and Andean peoples where the same figure represents the “Emu in the Sky” or “Yacana” (Llama), respectively (Gullberg et al., 2020). These figures could simply be marked by polygons, similar to DSO outlines which have recently been introduced.

2.3.3 Calendars

Every few weeks or months, there are questions in the Stellarium support channels which indicate difficulties of some users, even those working in historical research, with our common calendar.

First, the astronomical year counting used in Stellarium includes a year zero and negative signed years, while historical year counting knows only years labeled either AD (Anno Domini, also AC After Christ) or BC (Before Christ). Some authors also prefer to use CE, Common Era, and BCE, Before Common Era. Year 1BC is equivalent to astronomical year 0,2 BC=–1,… 100 BC=–99. The apparent offset of one between counting systems for years before AD 1 seems to confuse some users (Fig. 2.6).

Figure 2.6:

An apparently trivial calendar problem: counting years

Another major problem are the consequences of the Gregorian Calendar Reform, where October 4^th, 1582 was followed by October 15^th. While most Catholic countries soon (by about 1583) had adopted the new calendar, the Protestant and Orthodox countries ignored any Papal decrees and kept counting their dates in the Julian Calendar, leading to a calendrical rag rug and necessity for double dating over large parts of Europe that lasted for more than a century (Wilkins, 1961, section 14C) or even (in case of Russia) into the 20^th century. Presumably all widely available general purpose astronomy programs follow the official introduction date of the Gregorian and use the Julian calendar for all dates before and up to October 4^th, 1582. Evaluating or replaying observation records from Protestant countries may deserve extra attention about which calendar was in use.

Also, prehistoric dates are displayed in the proleptic Julian calendar, which shows its uncorrected errors the stronger the farther back we set a date. In the Neolithic (e. g., around –4500), summer solstice is displayed on a date in “late July”. Some users set the date “June 21^st” baked into their memories and ask whether the program is in error. Solar ecliptical longitude is the only help to find the key points of the year in these remote times. In contrast, Maya Long Count dates are often translated in the literature to dates in the proleptic Gregorian calendar, while replaying any records of pre-Columbian Maya observations in Stellarium (and other programs) would require setting Julian calendar dates.

A long-standing development wish has therefore been started recently by adding a first version of a “Calendars” plugin in Stellarium’s version 0.20.4 (December 2020) which should, extended over the next few releases, provide at least the multitude of calendars that have been made accessible algorithmically by Reingold & Dershowitz (2018). It displays the current date as written in a selection of these calendars in parallel, and allows direct operation with date elements from other cultures where the respective calendar allows setting a unique date.

2.3.4 Simulation of transient phenomena

Stellarium is a graphically intuitive tool to visualize transient astronomical phenomena like Solar and Lunar eclipses, comets, or also Novae and Supernovae (Zotti & Wolf, 2020b; Zotti et al., 2020b). Starting with version 0.21.0, Stellarium provides a visualisation of Earth’s shadow in Lunar distance in the form of circles outlining umbra and penumbra (Fig. 2.7).

Figure 2.7:

Simulation of Lunar eclipse 2018-07-27 in Stellarium 0.21.0

A new feature is the display of circles indicating borders of umbra (inner circle) and penumbra (outer circle), which are necessarily displaced from the Antisolar Point (ASP) in the ecliptic by effect of topocentric parallax. The foreground shows a (highly enlarged) part of the skyline seen from Vienna’s historical Kuffner observatory. The photo panorama has been made by Michael Prokosch and made available on Stellarium’s collection of “landscapes”. A “lightscape” was added by the author to simulate artificial light at night: bright windows, street lamps, red lights on high buildings, and a general “urban skyglow”.

Eclipses are important events which are frequently used in dating historical records. For the critical correction factor ΔT, which describes the irregular slowdown of Earth’s axial rotation, a multitude of models has been implemented, from which the user can select one or even make an own model following the common paraboloid development. Users should however always bear in mind that all known models are supported by observations that go back to about the 8^th century BC only, and therefore any conclusions derived from eclipses which can be simulated for much earlier times should be taken with increasing caution the earlier the simulated date lies.

2.3.5 Interfacing with other programs

During development of a major exhibition on Stonehenge (Zotti et al., 2017), an interface was developed which allows control of Stellarium with an HTTP interface, be it an HTML control interface served by the program or by external programs. This allowed developing the interface with the Unity game engine described in section 2.3.1, but is also already used by several third-party programs. With this interface, Stellarium can be operated as “astronomical simulation display window” for arbitrary other programs which just have to send date, time, coordinates or object to center the scene on, vertical field of view, and whichever coordinate grids, markers, object classes etc. shall be displayed. Programs can then retrieve astronomical data about particular objects.

Another way of interfacing is the configuration of several computers running Stellarium synchronized using the RemoteSync plugin. This may be useful to display one presenter’s screen to many clients (when projection is not possible), or, when synchronisation of particular features is inhibited in the menu, the display of different aspects of the same scene, like a parallel presentation of several skycultures, or of the appearance of the same object in a multitude of telescope/ocular combinations.

2.4 Conclusion

Stellarium is a highly popular desktop astronomy program with an immense variety of options and applications (Zotti & Wolf, 2020a; Zotti et al., 2020b). The current Stellarium development team is committed to solve the remaining astronomical accuracy issues and then get rid of the version number 0 which still indicates “astronomically unfinished”, and then extend the program further for research in topics relevant also to cultural and historical sciences.

Despite Stellarium’s open-source nature its main features are developed by only very few enthusiasts who also invest lots of their spare time into the program. Major and fast progress can be expected only from funded “daytime” projects. We invite contributions in code, data and ideas.

Acknowledgements

The Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology (https://archpro.lbg.ac.at) is based on an international cooperation of the Ludwig Boltzmann Gesellschaft (A), Amt der Niederösterreichischen Landesregierung (A), University of Vienna (A), TU Wien (A), ZAMG-Central Institute for Meteorology and Geodynamics (A), 7reasons (A), ArcTron 3D (D), LWL-Federal state archaeology of Westphalia-Lippe (D), NIKU-Norwegian Institute for Cultural Heritage (N) and Vestfold fylkeskommune-Kulturarv (N).

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