Moonrise and moonset

Moonrise and moonset are times when the upper limb of the Moon appears above the horizon and disappears below it, respectively. The exact times depend on the lunar phase and declination, as well as the observer's location. As viewed from outside the polar circles, the Moon, like all other celestial objects outside the circumpolar circle, rises from the eastern half of the horizon and sets into the western half^[1] due to Earth's rotation.^[2]

Direction and time

Direction

Since Earth rotates eastward, all celestial objects outside the circumpolar circle (including the Sun, Moon, and stars) rise in the east and set in the west^[2] for observers outside the polar circles. Seasonal variation means that they sometimes rise in the east-northeast or east-southeast, and sometimes set in the west-southwest or west-northwest.^[3]

Time

The Moon's position relative to Earth and the Sun determines the moonrise and moonset time. For example, a last quarter rises at midnight and sets at noon.^[4] A waning gibbous is best seen from late night to early morning.^[5] The Moon rises 30 to 70 minutes (should be a fixed number, about 50 minutes, if it's the same 13 degrees) later each day/night than the day/night before, due to the fact that the Moon moves 13 degrees every day. Hence, the Earth must move 13 degrees after completing one rotation for the Moon to be visible.^[6]

Moonrise/moonset for different moon phases

Lunar phase (illustration as seen from northern hemisphere)	Moonrise[a]	Culmination time (highest point)	Moonset	Best seen
New moon	Sunrise	Noon	Sunset	Not visible unless there is a solar eclipse
Waxing crescent	Late morning	Afternoon	Late evening	Late morning to early evening
First quarter	Noon	Sunset	Midnight	Early evening to late night
Waxing gibbous	Afternoon	Late evening	Predawn	Early evening[7] and most of night
Full moon	Sunset	Midnight	Sunrise	Sunset to sunrise (all night), a lunar eclipse is then possible
Waning gibbous	Late evening	Predawn	Late morning	Most of night and early morning[5]
Last quarter	Midnight[4]	Sunrise	Noon[4]	Predawn to post-sunrise
Waning crescent	Predawn	Late morning	Afternoon	Predawn to afternoon

Visual appearance

The Moon appears to be larger at moonrise or moonset due to an illusion. This illusion, known as the Moon illusion, is caused by an effect of the brain. There is no definitive explanation for the Moon illusion. However, it is most likely because of how the brain perceives objects at different distances, and/or the distance we expect objects to be from us when they are near the horizon.^[8]

The Moon appears to be more yellowish near the horizon. This is for the same reason the Sun and/or sky appears to be orangey-red at sunrise/sunset. When the Moon appears near the horizon, the light coming from it has to pass through more layers of atmosphere. This scatters the blue away, and leaves yellow, orange, and red.^[9] This is also the reason the Moon appears red during a deep partial or total lunar eclipse.^[10]

Notes

References

This page was last edited on 27 April 2024, at 02:49