What is the difference between angular size and linear size

Many deep sky objects such as galaxies and nebulas appear as non-circular and are thus typically given two measures of diameter: Major Diameter and Minor Diameter. Before we start talking about the idea of angular size we need to quickly review how we measure angles. There are degrees in a circle, but the degree is not the smallest unit we use to measure angles. If we divide each degree up into 60 equal slices, each slice represents an angle that is 1 arcminute across. If we further divide an arcminute up into 60 yet smaller evenly spaced slices, those slices each represent an angle that is 1 arcsecond across.

We can measure angles by specifying the number of degrees, arcminutes, and arcseconds that they span. Now we know how to measure an angle, we can begin talking about the angular sizes of objects. If you draw lines from you to each side of an object in the distance, the angle between those two lines is that the object's angular size.

This is illustrated in the diagram at the top of the page. There is a handy trick that you can use to estimate the angular size of something using nothing more than your hand. All you need to know is that if you hold your hand at arm's length, the distance across the end of your pinky finger spans an angle of about 1 degree. Let's try and calculate the angular size of the Moon.

All you need is a tape measure and a ruler. Hold the ruler at arm's length and measure the diameter of the Moon, you may have to wait for a full moon to be able to accurately measure it. You should get a reading of between 5mm and 8mm depending on the season and the length of your arm. Note down your measured apparent size of the Moon.

Next, you need to measure the distance between your hand and your eye you may need help with this one and note this down as well. Both measurements need to be the same units, ideally millimetres. Learn more. How to get apparent linear diameter from angular diameter Ask Question.

Asked 10 years, 1 month ago. Active 10 years, 1 month ago. Viewed 5k times. Add a comment. Active Oldest Votes. Shouldn't the visual size get smaller as the distance increases? I need to know what the apparent visual size will be at any given distance. The angular diameter does indeed change as the distance changes. As the distance increases, the apparent size decreases.

It is does not vary exactly as the inverse of the distance, but as the distance gets much larger than the actual size, it varies very close to the inverse of the distance. I know how to find the angular diameter. What I need to find is the visual diameter the size it appears from the viewpoint , measured in units, at any given distance. I appreciate you sticking with me on this, I hope that makes things more clear. The angular size, linear size and distance can be calculated using the formulas: Thus, we can determine the distance from the object if we knew its size and angular size.

Distance from angular size and linear size. Linear size. Calculation precision Digits after the decimal point: 2. Linear size from angular size and distance. Angular size from distance and linear size. Everyone who receives the link will be able to view this calculation.