Читать книгу Simple Stargazing - Anton Vamplew - Страница 17

Depending on the hemisphere you are in, either the Plough or the Southern Cross is easy to spot if you know in which direction to look and how big they are. This idea of size is useful to understand, so I’m going to take a moment to show you how to measure things in the sky.

Let’s start with the Moon. Most people would say that it is a lot bigger than it really looks. You may be surprised when you realise that the end of your little finger held at arm’s length easily covers the Moon – with room to spare. Have a go next time the Moon is out.

The Plough appears only slightly longer than your outstretched hand – though it depends on how big your hands are, of course.

Of course, you can cover different amounts of the sky using more of your hand, arm or even your feet if you’re fit enough. For now it is useful to know that the Plough, as viewed from the Earth, is slightly longer than your outstretched hand held at arm’s length. However, there are loads of tiny things to see, so it is time to get a little more scientific.

You probably know that if we want to divide any circle into smaller units, we use degrees – or, more accurately, angular degrees – and that 360 of them make up a full circle. If you imagine the circle as a clock, the minute hand moves through 360 degrees when it goes all the way round, which takes one hour.

A single degree is a very small measurement, equal to the barely visible movement that the minute hand on a clock makes in 10 seconds. But in space many objects are extremely small, so we need incredibly small units to measure with. The space boffins have therefore divided the degree into 60 smaller segments, and each one of those into a further 60 even smaller segments.

Unfortunately the names of these smaller segments sometimes lead to confusion, because the 60 smaller segments of an angular degree are called angular minutes (or arc minutes) and the 60 smaller segments of the angular minute are angular seconds (or arc seconds).

These units are represented by the following symbols:

° degrees

’ minutes

” seconds

You can avoid confusion simply by remembering that if you see the word angular or arc anywhere, the measurements are to do with angles, not time.

With all this information about measuring, let’s take a look at the sizes of some space things in degrees, arc minutes and arc seconds:

Thing	Approximate angular size
Distance from the Pointers in the Plough to Polaris	28°
Length of the Plough	24°
Your outstretched hand at arm’s length (roughly)	22°
Distance apart of the Crux Pointers	6°
Distance apart of the Pointers in the Plough	5°
Your forefinger at arm’s length	1°
Your little finger at arm’s length	½°
The Sun	½°
The Moon	½°
Distance of Ganymede from Jupiter (that’s the brightest of the planet’s main Moons)	6’
Resolution of the unaided eye (this means your eye can see two objects that are very close together as two objects rather than one.)	3’ 25”
Maximum size of Venus	1’
Biggest crater on the Moon	1’
Your eye can see single objects as small as… (about)	1’

What’s interesting from this table is that technically we can see at least one of the moons of Jupiter as well as the crescent-shape phase of Venus simply by gazing skyward with our unaided eyes (the 20–20 variety is required). However, in practice the usually super-brightness of Jupiter drowns out the fainter light from its moons, while the dazzling appearance of Venus does the same for its crescent.

Look! You can use many bits of your hand to measure different sizes in the sky.