Why do I sometimes see a horizontal half moon, instead of the more usual vertical one?... this is a half moon with its curve closer to the horizon, like a bowl of cereal. I would like to know why it only appears sometimes that way.
This is a very interesting observation about the moon; an absolutely awesome question. The cause of the change in orientation of the half moon is due to the seasonal change in the orientation of the earth to the sun. And this change is most apparent when we view the half moon as it sets. Further, changing our latitude will also change our view of the setting half moon.
To start, we should define the half moon in more direct terms so everyone is sure about our topic. The half moon we most commonly see is that phase astronomers call the moon in its first quarter. We see this phase generally from the afternoon and into the evening about one week after the very first appearance of the new-moon crescent. We also see the half moon as the moon in its last quarter. The appearance of this aspect is after midnight and into the morning and occurs about one week after the full moon, or about three weeks after the new-moon crescent. Interestingly, the first-quarter moon rises in the eastern sky about noon and sets about midnight, while the last-quarter moon rises about midnight and sets on the western horizon about noon. Throughout our discussion, first-quarter moon or last-quarter moon is the same as half moon.
Everybody knows that the moon changes phases throughout the course of about a month. The lunar month is about 29 days. The moon goes through one cycle of its phases from new moon, through full moon, and back to new moon again, in a lunar month. We can see these changes in the slightly different appearance of the moon every evening. This means about every 29 days, or so, we will see the moon in its first quarter.
The phase-angle of the moon is defined by the angle between the moon, the earth, and the sun. The part of the moon that is sunlit corresponds directly with this angle. This angle changes from day to day. Nevertheless, one particular phase of the moon, the half moon, occurs with special circumstances because the angle between the moon, the earth, and the sun, is 90 degrees - known in geometry as a right angle. Therefore, when we are seeing the moon in its first quarter, we are only seeing one quarter of the entire surface of the moon. We are looking at the moon in a direction that is at a right angle to, or perpendicular to, the direction of the sun. The half moon, as we call it, is one-half of the full moon. For the half moon, the angle between the earth, the moon, and the sun is 90 degrees.
If we are looking at the first-quarter moon, either when just rising or just setting, we will notice that the alignment of the lunar line of demarcation makes an angle with the horizon. During some months this line seems more steep, at others not quite so much. The lunar line of demarcation is the most important feature we see when viewing the setting half moon for getting a sense of the earth’s seasonal tilt toward, or away from, the sun. The lunar line of demarcation separates the sunlit side of the moon from the unlit side, and for the moon in its first or last quarter, we see this as a line that is straight, not curved.
Each year the tilt of the earth, with respect to the sun, changes by a total of slightly more than 46 degrees. During the northern hemisphere summer, the earth is tilted toward the sun reaching its maximum tilt about June 21 each year. On this date the noontime sun is directly over the Tropic of Cancer, about 23.4 degrees north latitude, because the north pole of the earth’s axis s tilted about 23.4 degrees toward the sun. The sun reaches its highest noontime position in the northern-hemisphere sky on this date. Then six months later, during the southern hemisphere summer, the sun reaches its maximum southerly noontime position in the southern-hemisphere sky about December 21 each year. On this date the noontime sun is over the Tropic of Capricorn, about 23.5 degrees south latitude, because the south pole of the earth’s axis of the earth is tilted 23.5 degrees toward the sun. Of course, at this December 21st time, the northern and southern hemispheres are in seasonal opposition; in the northern hemisphere we see the winter sun low in the southern noontime sky, but the southern hemisphere sees the summer sun at its highest in the noontime sky.
The moon, in its orbit around the earth, does not change its orientation to the sun. However, the seasonal tilt of the earth changes our perspective of the moon in the sky, causing the winter-to-summer, month-to-month appearance of the first-quarter moon to rotate counter-clockwise to a maximum position for the summer, then over the course of the next six months, to rotate clockwise a total of nearly 47 degrees back to an opposing maximum position for the winter.* This extreme counter-clockwise, then clockwise rotation, is most notable with our view of the first-quarter moon at the time when the moon is just setting. This seasonal rotation also happens with our view of the rising last-quarter moon. In this case, the sense of rotation is in the counterpart direction to that of the first-quarter moon. We can see this month-to-month, seasonal change in the orientation of the first-quarter moon, especially at the time the moon is setting, by noticing the change in the angle the half-moon makes with the horizon. During certain months the moon appears more horizontal, during other months more vertical. The half moon will reach its extreme positions when the half moon occurs closest to the northern-hemisphere's summer solstice or winter solstice. In the northern hemisphere near the summer solstice on June 21, the half moon stands more vertically; near the winter solstice on December 21 the half moon is more horizontal. The opposite is true for these dates in the southern hemisphere.
When we face the setting first-quarter moon, the sun is 90 degrees to our right, well below the horizon. The reason we can see this sense of rotation related to the earth’s seasonal tilt is because we see the setting moon at an angle that is 90 degrees away from the position sun, which is also at 90 degrees away from the direction of our seasonal tilt. We have a perpendicular view of the moon as, essentially, a fixed spatial frame-of-reference at exactly the most optimal time. This gives us our best advantage to see the consequences of earth’s seasonal tilt in relation to an unchanging reference, the moon. We do not see much of the consequences of the earth’s seasonal tilt when we look at the full moon because the full moon is directly facing us in opposition to the sun. The only sense we gain of earth’s seasonal tilt when viewing the full moon is the changing seasonal, or month to month, position of the full moon in the midnight sky.
The angle between the lunar line of demarcation and horizon at, or near, the time the first-quarter moon sets, can be easily determined on any date when the moon is in its first quarter, as the sum of the solar declination on that date and latitude of the observer. Solar declination is the measured angle from the plane of the equator to the directly overhead position of the sun exactly at solar noon. This angle is the same as the latitude at which the sun is directly overhead at solar noon. Declination is positive when the angle to the noontime position of the sun is north of the equator, and negative when the angle is to the south. However, we can see the resulting changes in the night-sky appearance of the setting first-quarter moon here.
Below are some dates and facts for using the link above. Times are given as astronomical local standard time (2400hr is a single instant in astronomical time, as the astronomical clock advances past 23hr 59min 59sec, midnight is 00hr 00min 00sec). North latitude is shown as positive in calculations, south latitude as negative. For the setting first-quarter moon, the sum of latitude and solar declination is the angle of the lunar line of demarcation with the horizon.
Some dates and astronomical local standard times for the setting first-quarter moon –
December 22 2020 00hr 00min (midnight)
June 18 2021 00hr 00min (midnight)
A location that can be used in sky chart above is Phoenix, AZ
lat. 34 degrees N long. 112 degrees W
Changing the latitude will change the aspect of the setting first quarter moon in relation to the horizon. Here are some facts to illustrate the changes by using the sky chart... (Try them!)
Fact: The same phase of the moon recurs about every 29 days, or so.
Fact: At the Arctic Circle on the first day of summer, the line of demarcation of the setting first-quarter moon is perpendicular (90 degrees) to the horizon. This angle is the sum of the latitude (+67 degrees N) and solar declination (+23 degrees) on this date, namely, 67 + 23 = 90.
Fact: At the Tropic of Cancer on the first day of winter (Northern Hemisphere), the line of demarcation of the setting first quarter moon is parallel (0 degrees) with the horizon. This angle is the sum of the latitude (+23 degrees N) and solar declination (-23 degrees) on this date, namely, 23 + (-23) = 0.
Any online ephemeris will give the solar declination for any day of the year. Now you can pick a month and a time to go outside and see the half moon rise or set.
This was a great question!