Why planets do not twinkle
Many points, changes that cancel.
Stars and planets
A planet is much closer to the earth than a star. Through a telescope or the naked eye it looks like a small disc, not a point.
Does a planet twinkle like a star?
What this lesson covers
The idea
Planets are much closer and are seen as extended sources, collections of many point sources whose variations in light average out to zero, so planets show no twinkling.
Stars and planets
A planet is much closer to the earth than a star. Through a telescope or the naked eye it looks like a small disc, not a point.
Does a planet twinkle like a star?
- No, it shines steadily
- Yes, even more
- It twinkles only at noon
Compare star and planet
Open the tabs. The numbers are only an illustration.
Averaging out
Planets are much closer to the earth, so they are seen as extended sources. Think of a planet as a collection of a large number of point-sized sources of light. The variation in the light from all the individual points averages out to zero, so the twinkling is cancelled.
Planets are much closer, so they are extended sources, a collection of many point sources; the total variation in light entering the eye averages out to zero, so planets do not twinkle.
Notes
A planet is a collection of many point sources; the variations in light average out to zero, so the twinkling is nullified.
Check yourself
Why are planets seen as extended sources?
A planet can be thought of as…
The total variation in the light from the points of a planet…
So the twinkling effect of a planet is…
- They are much farther away than the stars. The planets are much closer to the earth than the stars, and are thus seen as extended sources.
- They are much closer to the earth — correct. Yes!
- They are point-sized. The stars approximate point-sized sources. The planets are extended sources.
- one single point of light. A planet is considered a collection of a large number of point-sized sources of light.
- a source that gives no light. A planet is a collection of a large number of point-sized sources of light.
- a collection of a large number of point-sized sources of light — correct. Yes!
- averages out to zero — correct. Yes!
- adds up to a large flicker. The total variation in the amount of light entering our eye from all the individual point-sized sources averages out to zero.
- is the same as for a star. For a planet the variations average out to zero, so there is no twinkling.
- doubled. The variations average out to zero, which nullifies the twinkling effect.
- nullified — correct. Yes!
- stronger than that of a star. A star twinkles. For a planet the twinkling effect is nullified.