q & a

Here are some of the questions which visitors to Inconstant Moon have asked. They have been divided into six sections...

movement: the moon's orbit and phases
physical: form, characteristics and geology
phenomena: unusual lunar events
observation: what can be seen... and what can't
terminology: naming of the moon, phases and features
miscellaneous: anything not covered avove

If you cannot find the answer to your question, go ahead and ask it!

earth from the moon

q: If we were sitting on the moon, does the Earth appear to move across the sky? My friends all claim that it does because they have seen the pictures of earthrise from the Moon. I say that it does not move or moves very little, and that because we on Earth always see the same face of the Moon that the Earth does not appear to move across the sky from the Moon. (Josh Roberts, Washington, DC)
a: You'll be pleased to know that you are correct! An observer on the Moon will see the Earth move slightly in the sky, but it will always stay within a few degrees of the same spot. The photos of earthrise your friends mention are, I suspect, the ones taken from Apollo command modules orbiting the moon as they emerged from the lunar farside.

earth from the moon... again

q: I believe you've made an error in one of the reponses above. If one were sitting on the moon, looking toward the earth, it would be apparent tht the earth does not "stand still" with respect to the background stars. I'm fairly sure that the earth would wheel through the heavens with a periodicity equal to one synodic period, about 29.5 days. Not very fast, but certainly not staying within a few degrees of some fixed spot. (Johnie Spruiell, Southlake, Texas)
a: Celestial mechanics is not always intuitive, and I certainly do make mistakes, but on this occasion I stand by my answer. Lets tackle the problem in parts...

Imagine a line in space connecting the centre of the Earth with the centre of the Moon. During a period of about 24h, as the Earth rotated, the point where the line intersected the Earth's surface would describe a great circle crossing the equator twice and tilted a bit. Someone standing on that circle would see the Moon apparently passing overhead roughly once a day. In fact as the Earth rotates every 24h, the Moon revolves around it every 27 days - the sidereal month - so the intersection would actually take 24h 49m to return to the same spot.

The story on the Moon is different, though. It has captured rotation, so it spins on its axis with a period of 27d, exactly the same as the sidereal month. From the Earth we always see the same face of the Moon, and the centre-to-centre line would intersect the Moon's surface somewhere in the area of the Sinus Medii (selenographic latitude 0 and longitude 0)... always! Someone stood in this area would see the Earth directly overhead, the only variations being small periodic ones due to libration.

However, the patient observer on the Moon would indeed see something wheel through the heavens with a periodicity equal to one synodic period: the Sun. The combination of the Earth's 365 day journey around the Sun and the Moon's 27 day trip around the Earth produces a lunar day equal to 29.5 Earth days. Lunar noon in the Sinus Medii is essentially the same as the full moon seen from Earth, and the 29.5 period between lunar noons is the same thing as the synodic month from one full moon to the next.

rising, setting & phases

q: At what time of day or night does the moon rise in each of it's phases?
a: The new moon rises and sets together with the Sun. During a lunation of 29.5 days, as the Moon passes through a full cycle of phases, it rises roughly 50 minutes later each night. By first quarter it rises around midday and sets around midnight. When it reaches full it will be up from sunset to sunrise, more or less. And by last quarter it will rise at midnight and set at noon, roughly speaking, before, as it approaches another new moon, its rising and setting times converge again with those of the Sun.

moonrise and moonset pattern

q: I expected to find a pattern in the time between moonrise and moonset and also a pattern in the amount of time later moonrise and set occurred each day. I thought I could use the pattern to predict what time moonrise or set would occur. There seems to be no predictable pattern. Can you tell me why? (Nancy Dechant, Akron)
a: There is a pattern to the Moon's rising and setting, and in broad terms it can be seen fairly easily. However, the precise rules are extremely complicated, depending on the many components of the Moon's orbit and the Earth's geometry. The astronomer Thomas Maskelyne, notably, spent many years refining a mathematical model of the Moon's motion to produce accurate tables for use in navigation.

rising and setting

q: Does the moon rise every night at the same time and in the same location in the sky? (Will Jones, Concord, N.C.)
a: The Moon rises and sets in roughly the same places as the Sun, but about 50 minutes later each night.

Every 29-30 days there is a new moon - we cannot see it because it is near the bright Sun in the sky and rises and sets at the same times. A few days later we can see a thin crescent moon which sets just after sunset. Each successive night, as its phase gets greater (called waxing), it sets a bit later until 14-15 days after the new moon we see a full moon. The Sun and the Moon are on opposite sides of the Earth, and when one sets the other rises.

For the next 14-15 days the Moon's phase wanes back to a crescent which rises shortly ahead of the Sun, before the whole process repeats.

Like the Sun, the Moon also has seasonal variations in its movements. During the summer, when the Sun rises high into the sky, the new moon does the same. The points where it crosses the horizon (rising and setting) are further apart, and it reaches a higher point in the sky than during the winter when its rising and setting positions are closer together.

For the full moon, which is always opposite the Sun, the reverse is true. In summer the points on the horizon where it rises and sets are closer together, and while the Sun never goes very far below the horizon the full moon will never rise very far above it. During the winter it follows a longer arc through the sky, the shorter days and longer nights being matched by longer and higher full moons.

Many thanks to Mark Smith of Chapel Hill, NC for spotting the error contained in the original answer to this question.

orbital dance

q: Years ago I read an essay by Isaac Asimov suggesting that the moon, for various reasons including relative pulls of the earth and the sun, didn't actually go around the earth, but did a sort of cosmic dance around the sun together. The National Geographic also did an article asserting the same thing. What is the fact? (Tom Demmo, Dryden, NY)
a: Asimov is in fact correct. If you look at the Earth and Moon in isolation, the two revolve around a common barycentre, which is some 1750 km below the Earth's surface. However, if you plot their movement around the Sun, the two paths gently weave across each other to produce a "braided" pattern. Imagine two speed-skaters taking turns to overtake the other on the outside, roughly 12 times each per lap of the stadium.

monthly variation

q: How much does the 5-day-old moon vary from month to month? (Robert Pierrelee, Bossier City, LA)
a: The new moon might have occurred at any time during a 24 hour period, but the tours in Inconstant Moon (IM) are based on the assumption that it happened at midday. On average the terminator will advance just over 12° - equivalent to roughly triple the width of Copernicus - during one day. The maximum deviation from IM's assumed terminator position for any night is therefore about 6° - roughly the width of Ptolemaeus.

phase calculation

q: I am looking for an algorithm to calculate the Moon's phase. (Jacques Foucry, Paris)
a: There is a method for calculating co-longitude (the position of the Moon's terminator) in "Atlas of the Moon" by Antonín Rükl. The technique used was developed by the Dutch mathematician Jean Meeus, who has written several books of astronomical formulae and algorithms. Check out the IM Bookstore for either of these.

lunar months

q: Why is there a difference between the length of time between full moons (29.5 hrs) and the sidereal time for one orbit of the moon? Is it the rotation of the earth? (Mark, Griffin)
a: Imagine the physical arrangement of a full moon. A straight line drawn from the Sun through the Earth and then the Moon would continue on into, say, the constellation of Capricorn. One sidereal month later (27.3 days) the Moon has revolved around the Earth and returned to essentially the same position against the stars, so the Earth-Moon line points to Capricorn again. However, the Earth has revolved through roughly 1/12 of its orbit around the Sun, so the Sun-Earth line now points at Aquarius. The full moon will not occur until the Sun, Earth and Moon are once again in a straight line, so the Moon must revolve a further 1/12, roughly, around the Earth, until it appears in Aquarius. This slightly longer period (29.5 days) from full moon to full moon is called the synodic month.