Читать книгу Exploring the Solar System - Peter Bond - Страница 15

Since time immemorial, people have stared in wonder at the night sky. In previous millennia, when the darkness of the sky was not degraded by artificial lighting, it was easy to recognize how the stellar patterns drifted from horizon to horizon as the night progressed, and how they changed as the seasons passed.

However, in addition to the familiar, twinkling stars, observers noted seven objects that moved with varying speeds against the background of “fixed” stars.¹ In order of greatest apparent brightness, they were the Sun, Moon, Venus, Jupiter, Mars, Mercury, and Saturn. The ancient Greeks called them “planetes” (“wandering stars”), a designation we still use for all but the Sun and Moon.

For ancient astrologers and astronomers – the two disciplines were inextricably intertwined for many centuries – the most important of the wanderers were the Sun, which was responsible for daylight, and the Moon, which dominated the night. Both of these objects displayed visible disks and moved quite rapidly across the sky.

Careful study of their regular motions and apparitions enabled people to devise calendars and introduce convenient ways of measuring time. Thus, a year was the period before the Sun returned to the same place in the sky, while a month was the period that elapsed between each new or full Moon.

The other five planets were rather less noticeable, though each had its own peculiar characteristics. For example, Mercury and Venus never strayed far from the Sun in the twilight skies of morning or evening (Figure 1.1). The other three moved more slowly from constellation to constellation, sometimes describing loops in the sky as they appeared to temporarily reverse direction.

It was also evident that the seven planets often came together in the sky or even passed behind the Moon during occultations. They always remained within a narrow band on the sky, known as the zodiac (after the Greek word for “animal”). The Sun's annual path across the sky, called the ecliptic, ran along the center of this celestial highway. Clearly, the planes of the planets' orbits were closely aligned with each other.