Stars look a little higher

The idea

Starlight is refracted continuously by the atmosphere's gradually changing refractive index and bent towards the normal, so a star appears slightly higher than its actual position when viewed near the horizon.

Starlight comes down

Starlight travels through space and then enters the earth's atmosphere. The air gradually changes its refractive index.

What does the atmosphere do to the path of starlight?

• Bends it towards the normal
• Keeps it perfectly straight
• Stops it

Down through the air

Press Next step.

Apparent position

Starlight is refracted continuously as it passes through the atmosphere, whose refractive index changes gradually. The atmosphere bends starlight towards the normal. So the apparent position of the star is slightly different from its actual position: near the horizon the star seems slightly higher.

The atmosphere has a gradually changing refractive index and bends starlight towards the normal, so a star viewed near the horizon appears slightly higher than its actual position.

Notes

The atmosphere has a gradually changing refractive index and bends starlight towards the normal, so a star near the horizon looks slightly higher than it really is.

Check yourself

The atmosphere bends starlight…

Viewed near the horizon, a star appears…

The refractive index of the atmosphere, for starlight, is…

Starlight undergoes refraction…

• away from the normal. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position.
• not at all. Starlight undergoes refraction continuously on entering the earth's atmosphere.
• towards the normal — correct. Yes!
• slightly higher than its actual position — correct. Yes!
• slightly lower than its actual position. The star appears slightly higher (above) than its actual position when viewed near the horizon.
• exactly at its actual position. Because of atmospheric refraction, the apparent position is slightly different from the actual position.
• the same everywhere. Atmospheric refraction occurs in a medium of gradually changing refractive index.
• changing gradually — correct. Yes!
• zero. The medium has a gradually changing refractive index.
• only once, at the ground. The starlight, on entering the earth's atmosphere, undergoes refraction continuously before it reaches the earth.
• never. Starlight is refracted continuously as it passes through the atmosphere.
• continuously, before it reaches the earth — correct. Yes!