The apparent speed of the Moon's movement in the sky varies due to its elliptical trajectory around the Earth, which brings it closer or farther away from us at different times in its orbit.
The Moon takes about 27.3 days to complete one full orbit around the Earth: this is called the sidereal revolution. But be careful, to return to the same position relative to the Sun (for example, from one full moon to the next), it takes a little longer, about 29.5 days: this is the synodic revolution, in other words, the lunar phase cycle. This difference exists simply because while the Moon is making its orbit, the Earth is also moving along its orbit around the Sun. In short, these shifts mean that we do not observe exactly the same apparent speed of the Moon every night in our sky.
The Moon does not orbit the Earth in a perfect circle but describes a slightly elongated orbit called an ellipse. As a result, its speed changes depending on its position: when it gets closer to our planet (at perigee), it accelerates a bit, and conversely, when it moves away (at apogee), it slows down. In addition to this, the Moon orbits the Earth in a plane tilted differently from that of the Earth around the Sun, with about a 5-degree difference. This tilt explains why the Moon moves across the sky on varying paths over time, sometimes rising very high in the night sky or remaining quite low on the horizon. These two combined characteristics, elliptical shape and inclination, significantly alter its movement across the sky throughout the month and the seasons.
The Moon orbits the Earth, okay, but it still experiences a kind of gravitational "pull" from the Sun and other planets. The Sun, for example, exerts a significant attraction on the Moon, slightly altering its trajectory and speed depending on its position. This phenomenon, known as gravitational perturbation, causes noticeable variations in the pace of its movement across our sky. Even though this effect remains subtle, it explains why on some nights it seems to move faster. Additionally, when the large planets, like Jupiter or Saturn, come closer in alignment, their combined gravity adds a few minor irregularities to the apparent lunar speed.
This periodic anomaly actually refers to a regular variation in the apparent speed of the Moon across our sky. Due to the peculiarities of its orbit, our satellite sometimes moves slightly ahead or behind its expected average position. As a result, on certain nights, the Moon appears slightly early or lags behind compared to its usual pace. This cyclical variation spans about 27 days, corresponding to the time it takes the Moon to orbit the Earth. This shift directly influences its apparent speed in our sky, explains why it never strictly follows a precise schedule, and greatly contributes to the slight variations that amateur astronomers take into account to predict eclipses or certain star occultations.
The distance between the Earth and the Moon is not constant; it varies during the lunar orbit. The Moon orbits our planet in a slightly elliptical path, with a close point called perigee (about 356,500 km) and a far point named apogee (about 406,700 km). These variable distances directly influence its apparent speed in the sky: at perigee, it appears to move faster, while at apogee, it slows down. That's why, depending on its position in the orbit, you may notice subtle but real variations in the Moon's apparent movement over the nights.
Earth tides are not only influenced by the Moon but also by the Sun. When these two celestial bodies align during new and full moons, the tides are higher, a phenomenon known as "spring tides."
The regular visibility of the same lunar face is explained by the phenomenon known as synchronous rotation: the Moon takes as much time to rotate on its axis as it does to complete a full orbit around the Earth.
The lunar liberation is the optical effect that allows, over time, to observe about 59% of the Moon's surface from Earth, instead of the 50% one might imagine if the Moon always presented the same face.
A full moon known as a 'supermoon' appears up to 14% larger and 30% brighter than a normal full moon, due to its increased proximity when it passes at perigee, the closest point in its orbit around the Earth.
Lunar or solar eclipses do not occur every month because the lunar orbit is slightly tilted by 5 degrees relative to the plane of the Earth's ecliptic (its orbit around the Sun). Additionally, eclipses only happen when the Moon is precisely aligned with the Earth and the Sun, at the points where these orbits intersect (lunar nodes).
The elliptical orbit does not directly affect the lunar phases, which are due to the portion illuminated by the Sun. However, it influences the apparent duration of the phases, as the Moon moves more quickly when it is closer to the Earth (perigee) than when it is farther away (apogee).
The Moon's orbital cycle, called the sidereal period, is about 27.3 days. However, a synodic period, which is the duration between two consecutive full moons, lasts approximately 29.5 days.
Indirectly yes, since the position and orbital speed of the Moon determine its distance from the Earth, which slightly modifies the intensity of ocean tides. When the Moon approaches (perigee), the tides are stronger, a phenomenon known as perigean tides.
The apparent size of the Moon varies depending on the distance between the Earth and the Moon. When it is at perigee (the closest point), the Moon appears larger, a phenomenon commonly referred to as a 'Supermoon.'
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