Diameter = 3476 km
Moonrise and Harvest Moon
Between apogee and perigee, the Moon moves 12 ~ 15 degrees per day eastward among the star background. As a result, moonrise repeats every 24.3 ~ 25.2 hours, and so it must be delayed in successive days. There is always one day with no moonrise (e.g. 2007 July 08 in Hong Kong) and one day with no moonset (2007 July 23) in each lunar month.
The graph below shows the delay of moonrise during 2007 in Hong Kong. For example, the delay time changes from about 35 to 65 minutes and vice versa in each month in the second half of the year. This proves a very large variation of moonrise time in a lunation. The general saying that the Moon rises about 50 minutes later per day is not applicable most of the time.
Harvest Moon refers to the full moon that rises at minimum delay time during a year. It happens in few successive nights close to autumnal equinox (September 23) in the northern hemisphere. At these nights, the full moon rises around the time of sunset, and it appears only ~35 minutes later than the Moon did the day before. This is because at days close to autumnal equinox, the ecliptic, and hence the Moon's orbit, is at its least angle to the horizon at the time of moonrise in the northern latitudes. In higher latitudes (e.g. 500 N), the daily delay time of the Harvest Moon is even shortened to 20 minutes or less. Harvest Moon happens in March in the southern hemisphere. It is so named because the moonlight helps farmers to work at harvest time.
Tilting of the Moon's terminator, as seen from Earth
Between moonrise and moonset, the Moon in the sky seems to tilt differently from the vertical. This is because our eyes see the sky as its projection on the celestial sphere, and the Moon appears to move in a curved path above the horizon. Such perception is illustrated by a binocular watch on 2007 July 22 Hong Kong. At 3 pm, the first-quarter Moon was visible in daylight at an altitude of 30o. It was rising in south-east with the upper end of the terminator tilting to the observer's left hand side. At transit when the Moon was highest above horizon, the terminator looked almost vertical. Thereafter the Moon sank westward, and the terminator tilted to the right.
Tilting of the Moon's rotation axis, as seen from space
The Moon's rotation axis is tilted 1.540 from the vertical of the ecliptic (plane of the Earth's orbit). Hence the Sun always appears very low above the horizon at the polar regions of the Moon. Some craters at the poles are so deep that sunlight probably never reaches their bottoms. Based on the spectroscopic surveys by spacecraft, scientists speculate upon the existence of water-ice on the Moon (e.g. from impacts of water-bearing comets), especially an area of about 100 km diameter around the south pole which contains permanently shadowed depressions and where the temperature does not exceed -1800 C. So far traces of water-ice on the Moon are not affirmative. (http://science.nasa.gov/newhome/headlines/ast03sep99 1.htm)
Brightness of the Moon
Due to the elongation of the Moon from the Sun, the brightness of the Moon changes against its age in a lunation. Maximum brightness is at the full moon, equivalent to visual magnitude of about -12.7. Minimum brightness is at the crescents, see the following table. A moon-filter is sometimes needed to suppress the lunar brightness in visual observations.
Note that the eastern half of a Moon disc is slightly brighter than the western half, and that at ages approaching full illumination, the Moon rises in the afternoon and is naked-eye visible in daylight.
Moon Age (days)
Was this article helpful?