Key Takeaways:
- The Moon's visibility, including circumpolarity (remaining above the horizon) and invisibility, is determined by its declination, analogous to terrestrial latitude.
- A celestial object's declination dictates its visibility region; for example, if the Moon's declination is 10°, it won't be visible south of 80°S and will be circumpolar north of 80°N.
- The Moon's declination varies between approximately +28.7° and -28.7°, ensuring it rises and sets between latitudes 61.3°N and 61.3°S.
- The Moon's phase does not affect its circumpolarity or invisibility; only its declination is relevant to these phenomena.
Is there ever a time that the Moon does not set or rise, just like the Sun does from certain locations on Earth? Can it happen during any phase of the Moon?
Mark Narwa
Ottawa, Ontario
Yes, there are places on Earth where the Moon does not appear above the horizon. There are also places where the Moon is circumpolar and doesn’t set, at least for a period of time. To determine the locations where one would find the circumpolar Moon or see no Moon at all, we only need to know the Moon’s declination. Declination measures a celestial object’s angular distance relative to the celestial equator — the projection of Earth’s equator onto the sky. One could regard declination as the celestial equivalent of latitude. The declination of an object directly on the celestial equator, such as the Sun during either equinox, equals zero. The declination of the north celestial pole is 90° and that of the south celestial pole is –90°.
A celestial object’s declination determines its visibility region: where it’s visible, not visible, and circumpolar. For instance, the Sun’s declination on Nov. 12 is about –18°. Consequently, it will not be visible at any location north of 72° north, which is equal to 90 minus 18. Also, the Sun will be circumpolar at any location south of 72° south. On the winter (December) solstice, the Sun’s declination is about –23.5°. At this time, the Sun will not be visible at any location north of 66.5° north (the Arctic circle) and will remain circumpolar at any location south of 66.5° south (the Antarctic circle). On the summer (June) solstice, when the Sun’s declination is 23.5°, it will not be visible anywhere south of the Antarctic circle and will be circumpolar anywhere north of the Arctic circle. (Note: We are conveniently neglecting atmospheric refraction effects and the fact that the Sun is not merely a point in space.)
The same principle applies to the Moon or any other celestial object. Its declination determines its visibility region. For instance, if the Moon’s declination is 10°, it won’t be visible anywhere south of 80° south and will be circumpolar in any region north of 80° north. If you know the Moon’s declination, you can subtract it from 90 to determine its region of circumpolarity and also where it won’t rise above the horizon. Whereas the Sun is currently limited to a declination range between –23.5° and 23.5°, the Moon’s range is between about 28.7° and –28.7°. Therefore, it will always rise and set between the latitudes of 61.3° north and 61.3° south. Those of us at midlatitudes will always see the Moon rise and set, just as we always see the Sun rise and set each day.
This variation is independent of phase. For instance, the summer solstice Full Moon has a much lower declination than the winter solstice Full Moon. So, the Full Moon that occurs around the first day of summer will be circumpolar in many regions south of the Antarctic circle and the winter solstice Full Moon will be circumpolar in many regions north of the Arctic circle. All that matters is the Moon’s declination.
Edward Herrick-Gleason
Astronomy Educator, St. John’s, Newfoundland and Labrador
