Frequently Asked Questions
Everything you need to know about the Angular Diameter Calculator.
What is angular diameter?
The apparent angular size an object appears to have from a given distance, as opposed to its true physical size.
How does this calculator work?
Enter any two of true diameter, distance, and angular diameter, and it solves for the third using δ = 2·arctan(d/2D).
Is it free?
Completely free, no sign-up, downloads, or limits.
Why do the Moon and Sun appear the same size?
The Sun is about 400 times wider than the Moon but also about 400 times farther away, giving both an angular diameter of roughly 0.5°.
What units is angular diameter shown in?
Degrees, arcminutes, or arcseconds, chosen automatically based on scale.
Can I use this for distant stars?
Yes, results for very small angles are shown in arcseconds, the standard unit for stellar angular sizes.
What units can I use for true diameter and distance?
Kilometers, miles, astronomical units, or light-years.
What happens with an invalid distance?
Zero or negative distance is rejected with a clear message.
Is my data stored?
No; everything runs in your browser and isn't stored or sent anywhere.
Does it work on mobile?
Yes, fully responsive on all screen sizes.