In astronomy you’ll often see objects described as being so many degrees apart, or 3 arc minutes wide, or 45 arc seconds or so. What on Earth do all these numbers mean?
The entire sky is divided up into degrees, The whole sky is in fact 360 degrees. If you look from one horizon the opposite horizon, well that’s 180 degrees. The highest point in the sky is called the meridian, now look from the meridian down to the horizon, and…yes you’ve guessed it, 90 degrees. So objects such as the Moon, stars, and planets’ diameters and distances from each other in the sky can be described by using degrees.
Now these degrees of sky are divided down further, into smaller more precise increments called arc minutes. There are 60 arc minutes in each degree of sky. If you want to be even more precise, then these arc minutes are divided further into arc seconds, and there’s 60 arc seconds in each arc minute. But for planetary conjunctions you’ll usually only see planets distances from each other described in degrees and minutes. Or “Mars is 2.5 degrees from the crescent Moon tonight”, or “Saturn is 20 degrees above the horizon” for example. To give you another idea, on average the Moon is just under half a degree in diameter, about 31 arc minutes.
When it comes to the angular diameter of the planets, then as these are much smaller units, they are described in arc seconds. So on the Planets To See In The Sky Tonight page, you’ll see a planet’s angular size denoted as say, 35.7 arc seconds for example. This is it’s diameter in the sky as seen from you on Earth, and although the units are very small they give you an idea on how large the planet’s disk is, and whether it’s growing in size or reducing, as in nearer to Earth or further away.
To give you some idea of how much sky an arc second takes up, if you took a hair from your head, pulled it taught and held it at arms length…the width of that hair covers one arc second of the sky.
So here’s a real handy guide to measuring the night sky for yourself, (no pun intended). Using these illustrations, you can measure the angular size and distance of the sky. It is based on holding your hand out at arm’s length, can be used by anyone young or old, and is actually quite accurate.
Your little finger, 1 degree
Three fingers, 5 degrees
Back of your fist, 10 degrees
Index and little finger stretched as far as you can apart , 15 degrees
Your full hand span measures about 20 degrees
So now you can read astronomy information to measure the night sky any time, and know angular sizes and distances with confidence.
You can even test the “Moon illusion” with this method. The Moon’s size can appear a lot larger when it’s near the horizon, but in actual fact it’s no bigger than when it’s high in the sky. This is an optical illusion, and you’ll find that you can cover the Moon with the width of your little finger when it’s high overhead, or even when it’s looking huge on the horizon.