Find M31 in Binoculars
M31 – the Andromeda Galaxy – is the most distant object visible to the naked eye. Finding it in binoculars is a rewarding experience for new-comers to observing.
- Set the location to a mid-Northern latitude if necessary (M31 isn't always visible for Southern hemisphere observers). The UK is ideal.
- Find M31 and set the time so that the sky is dark enough to see it. The best time of year for this at Northern latitudes is Autumn/Winter, although there should be a chance to see it at some time of night throughout the year.
- Set the field of view to 6° (or the field of view of your binoculars if they're different. 6° is typical for 7x50 bins).
- Practise finding M31 from the bright stars in Cassiopeia and the constellation of Andromeda.
This part is not going to be possible for many people. First, you need a good night and a dark sky. In urban areas with a lot of light pollution it's going to be very hard to see Andromeda.
As described in section Handy Angles, your hand at arm's length provides a few useful estimates for angular size. It's useful to know if your handy angles are typical, and if not, what they are. The method here below is just one way to do it – feel free to use another method of your own construction!
Hold your hand at arm's length with your hand open – the tips of your thumb and little finger as far apart as you can comfortably hold them. Get a friend to measure the distance between your thumb and your eye, we'll call this D. There is a tendency to over-stretch the arm when someone is measuring it – try to keep the thumb-eye distance as it would be if you were looking at some distant object.
Without changing the shape of your hand, measure the distance between the tips of your thumb and little finger. It's probably easiest to mark their positions on a piece of paper and measure the distance between the marks, we'll call this d. Using some simple trigonometry, we can estimate the angular distance θ:
Repeat the process for the distance across a closed fist, three fingers and the tip of the little finger.
For example, for the author D=72 cm, d=21 cm, so:
Remember that handy angles are not very precise – depending on your posture at a given time the values may vary by a fair bit.
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