Why is the moon's terminator not at right angles to the sun?

Question

This evening I was watching the first quarter moon and the terminator was pointing straight down. The moon was well above the horizon but the sun had set and was well below. HOW? As the terminator is formed by thesunset/sunrise on the moon, the sun direction should be well above the horizon too.

Answer 1

This phenomenon is caused by observing the earth/sun/moon system from within the system itself i.e. from the earth's surface. If one were to take a spaceship and travel far from earth and look back it the moon it would be obvious that its lit side always pointed directly at the sun, as one would think. It would be easy to see this as one could be assured that all the lines-of-sight were straight lines.

From almost any viewpoint on the earth's surface, however, the sun appears to cross the sky in an arc. This is simply a consequence of living on a sphere: even if the earth's axis was not tilted, any point not at the equator will see the sun rise in the east and set in the west but because it would not pass directly overhead (it would only reach an altitude determined by the latitude of the observer) its path between east and west must be an arc. The same would be true for the moon, and the consequence of the sun and moon being constrained to cross the sky in an arc as opposed to a straight line is that, as the asker states, if you observe them at the same time the lit part of the moon will not appear to be pointing at the sun - you will tend to forget that the line-of-sight between them is actually curved as seen from the earth. However, if you took a photo of the sun setting and then one of the moon when it was about to set and compared them you would see that the lit part of the moon was indeed pointing to where the sun had set earlier: this is a valid comparison because they would both be on the same part of the arc. A similar effect is also very noticeable when the moon is a crescent - when it rises the "horns" of the crescent are pointing downwards, when it reaches maximum altitude they are pointing directly sideways and when it sets they are pointing upwards.

The fact that the earth's axis is indeed tilted does not affect the basic argument above, just complicates it a little because now even at the equator the path of the sun & moon will usually be an arc: it is only a straight line [overhead] at the equinoxes (and is so at one of the tropics on each solstice).

The slight tilt between the orbit of the moon and that of the earth is not the cause of this problem, as some answerers have stated, nor is the tilt of the earth's axis: these only affect the exact position of the moon in the sky relative to the sun i.e. which arc it traverses. Whether in a higher or lower arc, the moon will still appear to rotate as it crosses the sky in order to maintain its orientation relative to the sun and will still point directly at the sunset postion upon "moonset".

Answer 2

The moon's terminator is always 90degrees to the sun, relative to your observation point on the Earth. The reason it appears the way you describe is because the Sun, Earth and Moon are not in a direct straight line with each other. They form a triangle with the Earth at one corner, the Sun at another and the Moon at the other. Also the Earth is tilted slightly (about 5degrees) and this adds to the effect.

Answer 3

The apparently straight terminator implies that the Sun is 90 degrees from the Moon in the sky, but it tells you nothing at all about the location of the horizon. The great circle of the horizon line can intersect that 90° arc at any point, or not intersect it at all.

Answer 4

The terminator is a circle on the surface of a sphere. This circle lies in a plane that is perpendicular to the moon-to-sun line. When the moon, sun and earth form a right triangle, with the moon at the right angle, you are viewing the terminator edgewise (i.e., you are in the same plane as the terminator) so you see it as a straight line perpendicular to the moon-to-sun line. Otherwise it appears as part of a circle.

Answer 5

If you were viewing the terminator edge-on it would be a straight line perp. to the moon-sun line (which is almost parallel to the earth-sun line). However the only time we see the terminator edge-on and as a straight line is at a half moon, when you would see perpendicularity. This is related to the orbital tilt noted in another answer.

Answer 6

The orbital plane of earth and sun are at 5 degrees apart.

Answer 7

what happened is like the guy said the vector is not exactly 90 degrees to Uranus.