Angular Size Calculator
Calculate the angular size of any object as seen from any distance. This fundamental tool helps astronomers determine how large celestial objects appear in the sky and plan observations accordingly.
How to Use the Angular Size Calculator
- Enter the actual physical size of the object.
- Select the appropriate size unit.
- Enter the distance to the object.
- Select the distance unit and desired result format.
- See the calculated angular size with explanations.
Common Angular Sizes
| Object | Size | Distance | Angular Size |
|---|---|---|---|
| Sun | 1.39 million km | 1 AU | 32.0' (0.53°) |
| Moon | 3,474 km | 384,400 km | 31.1' (0.52°) |
| Jupiter | 142,984 km | 4.2 AU (closest) | 50.1" |
| Saturn | 120,536 km | 8.0 AU (closest) | 20.1" |
| Andromeda Galaxy | 220,000 ly | 2.5 million ly | 3.18° (191') |
| Orion Nebula | 24 ly | 1,344 ly | 66' (1.1°) |
Angular Measurement Units
Degrees (°)
The largest angular unit. The full sky is 360°, and the Moon appears about 0.5° across. Your fist at arm's length covers about 10°.
Arcminutes (')
There are 60 arcminutes in one degree. The Moon's diameter is about 30 arcminutes. Many star clusters and nebulae are measured in arcminutes.
Arcseconds (")
There are 60 arcseconds in one arcminute, or 3,600 in one degree. Most planets, double stars, and small deep-sky objects are measured in arcseconds.
Formula and Applications
The angular size formula is: Angular Size = 2 × arctan(Object Size / (2 × Distance))
For small angles (most astronomical objects), this simplifies to: Angular Size ≈ Object Size / Distance (in radians)
Applications include:
- Observation Planning: Determine if objects will fit in telescope field of view
- Photography: Plan framing and composition for astrophotography
- Solar System: Calculate planet sizes and moon phases
- Deep Sky: Estimate apparent sizes of galaxies, nebulae, and clusters
FAQ
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What is angular size in astronomy?
Angular size is how large an object appears in the sky, measured in degrees, arcminutes, or arcseconds. It depends on both the object's actual size and distance.
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How big does the Moon appear in the sky?
The Moon has an angular size of about 0.5 degrees or 30 arcminutes, which is roughly the size of your thumbnail at arm's length.
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Why do closer objects appear larger?
Angular size decreases with distance. The same object appears smaller when farther away, following the inverse relationship with distance.
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What units are used for angular measurements?
Angular sizes are measured in degrees (°), arcminutes ('), and arcseconds ("). There are 60 arcminutes in a degree and 60 arcseconds in an arcminute.