A full moon rises at approximately the same time the sun sets. This timing is a direct consequence of the alignment of the Sun, Earth, and Moon during this lunar phase.
A full moon occurs when the Moon is positioned on the opposite side of the Earth from the Sun, appearing fully illuminated to observers on Earth. While the Moon is technically “full” for only an instant, it appears completely round and bright for about a day before and after this moment.
The Orbital Mechanics That Dictate the Rise Time
The reason the full moon rises near sunset is rooted in a celestial arrangement known as opposition. Opposition means the Moon is positioned roughly 180 degrees away from the Sun in the sky, with the Earth situated between the two bodies. This geometry allows the entire face of the Moon visible from Earth to be bathed in sunlight.
As the Sun sinks below the western horizon, the Moon simultaneously appears above the eastern horizon. The Earth’s rotation causes the Sun to set in the west and the Moon to rise in the east. This 180-degree separation ensures that as one celestial body disappears, the other is coming into view.
This alignment ensures the full moon remains visible throughout the entire night, rising as the Sun sets and setting as the Sun rises the following morning.
Understanding the Daily Lunar Delay
The phrase “at approximately” sunset is necessary because the Moon’s rise time does not align perfectly every day. The Moon is constantly moving eastward in its orbit around the Earth, completing one full circuit in about 27.3 days. This orbital motion causes a noticeable lag in the time it takes for the Moon to reappear at the same position in the sky each subsequent night.
In a 24-hour period, the Moon advances about 12 to 13 degrees along its orbit. Since the Earth rotates 360 degrees in about 24 hours, it must rotate an extra amount to “catch up” to the Moon’s new, more easterly position. This extra rotation time results in the Moon rising, on average, about 50 minutes later each day than the day before.
This delay means that on the night of the precise full moon, the rise time is closest to sunset. On the night after the full moon, the Moon will rise roughly 50 minutes after sunset, and on the night before, it rises roughly 50 minutes before sunset. The exact delay can range from 30 to 70 minutes, varying due to the elliptical shape and tilt of the Moon’s orbit.
Seasonal and latitudinal differences also subtly alter the precision of this 50-minute delay. The Moon’s orbital path is tilted about 5.1 degrees relative to the Earth’s orbit around the Sun, which causes the Moon’s track across the sky to change throughout the year. Depending on the observer’s latitude and the time of year, the angle of the Moon’s orbit relative to the horizon can be steep or shallow, which affects the rate at which its rise time changes daily.
Comparing Rise Times of Major Lunar Phases
The rise times of the major lunar phases are a consequence of the changing angle between the Sun and Moon as observed from Earth. During the New Moon phase, the Moon is almost directly between the Earth and the Sun, making it invisible to us. Consequently, the New Moon rises and sets at roughly the same time as the Sun, appearing in the sky during the day.
The First Quarter Moon occurs when the Moon has completed one-quarter of its orbit and is positioned 90 degrees away from the Sun. This angle means that the First Quarter Moon rises around noon and sets around midnight. It is high in the sky during the early evening.
Conversely, the Third Quarter Moon is three-quarters of the way through the cycle and is also at a 90-degree angle to the Sun. This phase rises around midnight and sets around noon the following day. The Third Quarter Moon is best viewed in the hours leading up to sunrise.