A great deal of people seem to believe the Moon is only up at night.  I think it is because the crescent is used as a symbol for night-time.  Neither do they spend much attention at what one can see in the sky.  This can make them wonder how it is at all possible when they happen to see the Moon in daylight.  Which one can do regularly if one has lucky weather.

In reality the Moon orbits the Earth with an orbital period of 29½ days.  This applies in relation to the Sun and not in relation to the stars.  However, it is the relation to the Sun which matters to the phases.  Different parts of the Moon where it is day for the time being are directed towards the Earth.  Then people may see different phases in the places where the Moon is visible.

When the Moon rises and sets varies between its phases.  There is a point in its orbit when only its night-side faces the Earth.  Then it both rises and sets at the same time as the Sun does.  But we can’t see it if it don’t happen to pass more or less in front of the Sun.  The next day the times have been delayed and a thin crescent is visible at dusk.  For each night the crescent then grows and sets more lately.  The half-moon rises at the same time as the Sun is as highest.  At the opposite time of the day it sets.  It shines strong enough to also be visible in daylight.

During the following days what one sees of the Moon becomes larger and larger.  The times it rises and sets becomes later and later too.  Eventually comes a day with full-moon when the part of the Moon we can see is maximal.  It rises when the Sun sets and sets when the Sun rises.  After this the part of the Moon one can see gradually decreases.  However, the times of the Moon’s rising and setting continues to be delayed forward.  When less than half the dayside is directed towards the Earth it is no longer visible in daylight.  After 29 – 30 days the crescent is no longer visible.  Then the cycle starts anew.

Schemes showing the Moon’s phases does not show how the Moon leans.  It leans more and more the closer to the Equator one gets.  At the Equator the boundary between the day and night-side is horizontal.  In the southern hemisphere the Moon’s phases changes in the opposite direction from the northern hemisphere.  This is because people from different hemispheres see the Moon from different angels.  In contrast the Moon always faces approximately the same side towards the Earth.  This is called tidally locked rotation.  The orbital and rotation periods are the same for the Moon.

All times for the Moon’s rising and setting are just approximate.  The Moon’s orbit is a bit elongated.  Moreover, it leans 6 2/3 degree from the Earth’s orbit around the Sun.  Usually the Moon passes a little too far north or south for a solar or lunar eclipse.  As a result they can only take place during two short times of the year.  In addition the Earth’s orbital period is not an even number of the Moon’s.  This makes the two time periods happen a little earlier every year.  Lunar eclipses are visible everywhere the Moon is up.  Solar eclipses are only visible right were the Moon’s shadow meets the Earth.  The Moon can only cover the Sun entirely for at most a few minutes.

Finally, it is a coincidence that the Sun and Moon looks the same size.  Since the Moon’s orbit is a little elongated the similarity is not exact either.  How large the Moon looks varies by as much as 12%.  (This should not be confused with the moon illusion which is caused by faulty expectation.)  For the Sun it varies too, but only by 3,5%.  The consequence of these variations is that annual solar eclipses occur.  When the Moon passes exactly in front of the Sun it can’t cover it completely since it is too far away.  The Sun then looks as a glowing ring at maximum.  I think this is less common than the Moon covering the Sun completely.


Uploaded on the 26th of April 2024.