# Angular Size

1. Sep 23, 2004

### Gillipsee

How do you determine angular size from an object such as an asteroid? I tried searching the web but there were no real answers out there.
Help Anyone?

2. Sep 24, 2004

### Nereid

Staff Emeritus
Welcome to Physics Forums Gillipsee!

I'm not sure that I understand your question. It's very easy to measure an angle, and so measuring the 'angular size' of an object in the sky is also easy.

For example, you could bolt a small finder scope (such as the guide scope on an amateur telescope, or the 'telescopic sight' of a rifle) onto a large protractor. You note the angle when the cross-hairs are on one side of the object, and note it when they are on the other side; the difference is the 'angular size' of the object.

A common way to measure small angles, such as a minute of arc or smaller, is to take a picture. The linear scale on the picture can then be translated into an angular size, by using the scale of the image (you get this by taking a picture of an object of known angular size, or by analysing the optics of your camera).

For asteroids, it's a little trickier. For starters, no asteroid has an angular size greater than the best 'seeing' (the size of the blur that is what you see when you look at a star, or other point source, through a telescope) - except when it's a large asteroid about to hit the Earth! However, with adaptive optics, ground-based telescopes can take images of asteroids that have better resolution than 'seeing', so some asteroids' angular sizes can be measured from images.

However, the most common method is stellar occultation - the asteroid passes between us and a distant star, and we see a drop in the brightness of the star. By measuring how long the occultation takes, knowing where you are, and having a good orbit for the asteroid, you can work out the actual size of the asteroid (and then calculating its angular size is a piece of cake). Of course, it's best to have occultation observations from several different locations, across the occultation track, to determine the 'shape' of the asteroid!

3. Sep 24, 2004

### tony873004

$$AngularSize = (Diameter / (\pi Distance)) * 180$$