I read that the apparent motion of the moon is approximately its diameter every hour. However, we just observed the moon appearing to rise through a stationary cloud in about 10 minutes, it definitely appeared that the moon moved through its diameter as it rose far faster.

My son thought this was some kind of lensing effect at the horizon, but that makes the moon appear larger at the horizon, and this seemed far too fast.

If anything what we read is that the moon should appear to move slower near the horizon. As far as we could tell, the cloud was stationary, and we watched the moon steadily get covered until it was completely hidden behind the cloud.

Obviously the moon moves at different rates depending on its distance to the earth, but that is a tiny figure. This must have been some optical illusion.

  • 2
    $\begingroup$ The sky is about 360 moon-diameters across, but the moon certainly doesn't take 15 days to cross the sky. $\endgroup$ Aug 1 at 14:46
  • 4
    $\begingroup$ but that makes the moon appear larger at the horizon — no it doesn't: to the contrary, it makes the moon smaller in vertical direction, so that a disk becomes flattened to an ellipse. The fact that it seems larger (but doesn't "appear", since e.g. if you take a photo, you'll notice it's the same angular width as in the zenith) is known as the Moon illusion. $\endgroup$
    – Ruslan
    Aug 1 at 18:58

1 Answer 1


You are confusing two different motions:

  • Celestial objects appear to move 15 degrees per hour from east to west due to the Earth's rotation.
  • The Moon appears to move approximately 0.5 degrees (its diameter) per hour west to east relative to the stars due to the Moon's orbit around the Earth.

The motion you were "measuring" with the cloud was the 15 degrees per hour. A few minutes to cover the Moon's diameter is reasonable.

Note: The rate is 15 degrees per hour in motion in right ascension, and is only accurate for how far an object moves in the sky when the object is near the celestial equator. The actual motion also depends on the cosine of the declination. (Polaris and sigma Octans, the pole stars, also move at 15 degrees per hour in right ascension, but being near the poles, they hardly move in the sky; that is, cosine(90)=0.)

In the case of the Moon rise, the Moon is generally rising at some angle instead of "straight" up from the horizon or cloud line. In other words, the Moon is moving "up and right" for example. This also adds to the time it would take to move its apparent diameter.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .