I’m used to seeing and thinking of the moon phases as cutting the moon in left and right pieces from my perspective (New York) but tonight the moon seemed to be split by the shadow into top and bottom. Why would it appear to be sideways?
It was setting.
If it’s at its arc’s high point, it should be due south of you and lit on its east or west.
When it’s rising, it will be in the east & lit on top (if visible in daylight) or bottom.
When it’s setting, it will be in the west and lit on top (if visible in daylight) or bottom.
The bright side is always towards the Sun; the dark side is always away from the Sun. If you draw a line between the horns of the crescent or the equivalent points where the Moon’s outline circle first becomes flattened, then draw a perpendicular line to that one, the Sun will be somewhere along the plane formed by your eyes & that perpendicular line. The Sun might be somewhere below ground from your point of view, but it’ll be along that plane.
As you no doubt know, the Sun rises & sets pretty far South this time of year. Conversely, in mid-summer it’s pretty far North. The annual difference of about 46 degrees causes the orientation of the flat spot on a, say, half-moon to rotate one way or the other by a total swing of 46 degrees.
The Moon’s orbit is offset from the Earth/Sun orbital relationship by about 5 degrees. Sometimes that 5 degrees adds to the seasonal 46 degree variation and sometimes it subtracts. The sum of al this is the Moon can “rock back & forth” a total of about 57 degrees over the course of a year.
All of this means that at this time of year the Moon appears more “tilted” than is typical year round. And it seems you’ve just now noticed this.
It really just depends on where the moon is at in its orbit. Since it just past its waxing first quarter, it’ll be highest in the sky as the sun is setting. Now, as dusk turns into night, and the moon begins to head toward the west horizon to set, its orientation on the sky will seem to twist as the earth rotates.
It mostly has everything to do with where you are relative to the sun and moon. To get a better grasp, draw a perpendicular line off the center of the moon’s terminator. This will point you to the sun’s location. if its a crescent, or gibbous moon, just draw an imaginary line from its “horns” first.
ETA: Or what LSLGuy said.
I first said it was rising, then second-guessed myself, based on the present phase. But if you saw it in early evening last night, it was rising.
I believe you were right the first time, it was just past first-quarter, which would put the moon just past the first week of new moon, so the moon in NY would be rising around 2 or 3 pm (seemingly lit at its “top”) and setting about 12 hrs later, maybe 2 or 3 am (seemingly lit at its “bottom”).
It was about 1am, lit from the bottom.
I plugged your data into Stellarium and took a screenshot. It’s not that unusual. The first quarter moon sets around midnight or in your case, 1am when the sun is on the opposite side of the Earth and as noted up-thread, the lit half is the side facing the sun.
A first (or last) quarter moon near the winter solstice will appear more “bottom-lit” because the ecliptic “bends” southward between the Moon (declination close to 0 degrees) and the Sun (declination close to -23 degrees). The imaginary line between Moon and Sun, for a mid-Northern observer, is more perpendicular to the horizon.