Purpose: Get a feel for angular size and scale, and what they mean.
Background: Astronomers use degrees to specify both the location and the size of objects in the skies. To a person observing from Earth, the heavens form a dome, 360 degrees around the outside edge, or circumference, and 180 degrees from one horizon to the other, through the highest point, the zenith. An object's apparent, or angular, size is given by the angle it takes up moving parallel to the horizon and the number of degrees moving perpendicular to the horizon.
Degrees are divided in to tenths and hundredths, or into arc minutes and arc seconds, with 60 arc minutes in a degree, and 60 arc seconds in an arc minute. One can express the same angular height as 10.2 degrees, or 10 degrees, 12 arc minutes. The Sun and Moon, which are the largest naked- eye objects in the sky, each appear to be about half a degree. Angular size is not the same as physical size, and the angular size of an object changes as its distance from the observer changes. This may happen in a short time with cars and even pedestrians on Earth, but not so much with celestial objects!
Suppose we want to find and measure the angular size of an extended object like a galaxy. The angular size of a patch of sky where we look can be very important. If we use a telescope to examine an area of large angular size, like 20 x 20 degrees, the galaxy may almost disappear, like a needle in a haystack. A whole galaxy can look like just another star. In fact galaxies weren't discovered until well after the invention of the telescope for this reason.
If we get a better telescope (increase the magnification), we are examining a smaller patch of sky in greater detail. This is fine as long as the area still includes the galaxy. Then the galaxy occupies more of the image. It's the same when you zoom in on a friend with a camera lens; the photograph is the same size, but you get a "close up." Can an image have too much magnification? Yes, because the size of the film limits the size of the picture: zoom in too much and you might see only your friend's nose! In astronomy too, the size of the telescope's field of view (lens), and the light detecting device (film) attached to it, are limited.
In this exercise, we will vary the angular size and magnification of an image, keeping the physical size of the image the same. This is like the photography example above, in which the photographer is able to change the distance to his subject. This is not entirely realistic, as our distance from most large objects like Andromeda does not change. The image producing engine we will use is able to simulate this effect by manipulating the pixels of large images.
As a general rule, the larger the angular area of sky that is visible in your image, the lower the magnification. The higher the magnification, the smaller the area of sky shown in the image. Your goal is to determine the best magnification for measuring the angular size of the object Andromeda, and to estimate its angular size.
Your image should appear after a few seconds in a new window. Write down your answers to these questions:
6. Go Back to step 3 and change the size to 1.0 x 1.0 degree, and get a new image.
7. Go Back and change the size to 3.0 degree by 3.0 degrees.