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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[edit]

Direction[edit]

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] This north-south variation of the point along the horizon is bookended by two lunar standstills or turnarounds, the directions of which are sometimes depicted in archaeoastronomical constructions.^[4] It takes 18.6 years for the Moon to traverse this variation viewed from a vantage point on Earth.

Time[edit]

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.^[5] A waning gibbous is best seen from late night to early morning.^[6] 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.^[7]

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	6AM	Noon	6PM	Not visible unless there is a solar eclipse
Waxing crescent	9AM	Afternoon	9PM	Late morning to early evening
First quarter	12PM	Sunset	12AM	Early evening to late night, it looks like a half circle.
Waxing gibbous	3PM	Late evening	3AM	Early evening[8] and most of night
Full moon	6PM	Midnight	6AM	Sunset to sunrise (all night), a lunar eclipse is then possible
Waning gibbous	9PM	Predawn	9AM	Most of night and early morning[6]
Last quarter	12AM	Sunrise	12PM	Predawn to post-sunrise
Waning crescent	3AM	Late morning	3PM	Predawn to afternoon

Visual appearance[edit]

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.^[9]

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.^[10] This is also the reason the Moon appears red during a deep partial or total lunar eclipse.^[11]

Notes[edit]

References[edit]