Moon's Daily Journey: Degrees Traveled

The Moon moves approximately 13.2 degrees per day, or 92 degrees per week, relative to the stars. This means that from night to night, the Moon moves more than one hand-width to the east, relative to the stars, and about one diameter to the east each hour. The Moon's eastward motion is much slower than the sky's westward motion, so it rises later each day. The Moon's orbit is elliptical and not centred on the Earth, but on a point about 12,000 miles from the Earth's centre. As a result, the Moon's distance varies by 24,000 miles during each orbit.

The Moon moves 13.2 degrees per day

The Moon's eastward motion is much slower than the sky's westward motion. So, although the Moon moves east from day to day, it still has a net motion toward the west each day. This means that it rises in the east and sets in the west like the stars, but a little later each day. The Moon's orbit is elliptical and is not centred on the Earth, but on a point about 12,000 miles from the centre of the Earth. As a result, during each orbit, the Moon's distance varies by 24,000 miles. During half of its orbit, it is approaching us, and during the other half, it is moving away.

During the half of the Moon's orbit that it is approaching us, the mutual gravitational pull of the Earth-Moon system accelerates the Moon, causing it to move faster, until at the closest point in its orbit, called its orbital perigee, the Moon is moving about 6% faster than its average motion. Similarly, during the half of the Moon’s orbit that it is moving away from us, our mutual gravitational pull decelerates the Moon, causing it to move slower, until at the furthest point in the orbit, called its orbital apogee, the Moon is moving about 6% slower than its average motion. In addition to these actual changes in velocity, there is an apparent change in the Moon’s motion caused by the fact that the Moon is closer or further from us. When it is closer to the Earth, any motion looks faster in angular terms than when it is farther away. This effect causes another 6% apparent increase or decrease in the apparent velocity of the Moon’s motion in the sky, in addition to the actual change.

The Moon's apparent motion across the sky is measured in degrees per hour, just like the movement of celestial objects. On average, the Moon moves across the sky at approximately 0.5 degrees per hour. The Moon's orbit moves the Moon about 360 degrees around the Earth every 27-30 days.

The Moon's orbit is elliptical

The Moon's elliptical orbit is influenced by various factors, including the gravitational forces between the Sun, Moon, and Earth. The Sun plays a dominant role in perturbing the Moon's motion due to its greater mass. Additionally, the ever-changing distances and relative positions between these celestial bodies, as well as the inclination of the Moon's orbit and the oblateness of Earth, contribute to the complexity of the Moon's orbit.

The Moon's orbit is also inclined by about 5.1 degrees with respect to the ecliptic plane, which is the plane of Earth's orbit around the Sun. This inclination is not fixed and undergoes a slow eastward rotation, known as apsidal precession. The Moon's orbital inclination has an impact on the occurrence of eclipses, with eclipses taking place when the nodes of the Moon's orbit align with the Sun.

The Moon's elliptical orbit is also affected by a phenomenon known as tidal circularization, which acts over long timescales to reduce the eccentricity of its orbit and make it more circular. The Moon's orbit is notably more eccentric compared to the orbits of other moons in the Solar System, and this is due to its large size and proximity to the Sun.

The Moon's orbit is indeed a complex and fascinating aspect of our celestial neighbour, and its elliptical shape plays a crucial role in the dynamics of the Earth-Moon system.

The Moon's eastward motion is slower than the sky's westward motion

The Moon's motion is a fascinating aspect of astronomy. On average, the Moon moves 13.2 degrees per day, or 92 degrees per week, relative to the stars. This motion is eastward, in the direction of its orbit around the Earth. However, it is important to note that this motion is slower than the sky's westward motion.

The Moon's eastward motion around the Earth occurs approximately once every 27.3 days, which is known as its sidereal period of revolution or a sidereal month. During this time, the Moon's position changes relative to the stars, as it completes a full circle of 360 degrees. This eastward motion results in the Moon rising and setting later each day.

On the other hand, the sky's westward motion is caused by the Earth's rotation on its axis. The Earth rotates from west to east, causing the sky and celestial objects to appear to move in the opposite direction, from east to west. This westward motion is faster than the Moon's eastward motion.

The difference in speed between the Moon's eastward motion and the sky's westward motion has several interesting consequences. Firstly, it means that the Moon rises and sets like other celestial bodies, despite its eastward motion. Secondly, it leads to a delay in the Moon's rising and setting times each day. On average, the Moon rises and sets about 50 minutes later each day. This delay accumulates over approximately 27 days, after which the Moon returns to its original position in the sky and rises and sets at its usual times.

The Moon's eastward motion, slower than the sky's westward motion, is a key aspect of its complex dance with the Earth and the stars. This interplay of motions shapes the Moon's behaviour and contributes to the ever-changing celestial ballet we observe in our night skies.

The Moon's orbit is not centred on the Earth

The Moon's orbit and its rotation are connected. The Moon rotates at the same rate as its orbital motion, a phenomenon known as synchronous rotation. This is why the Moon always keeps the same face towards Earth. The Moon's orbit is also closely tied to the Earth's orbit around the Sun. The Moon performs the same sidereal cycle month after month, no matter where the Earth is in its orbit around the Sun.

The Moon's orbit is not perfectly uniform. Its motion varies due to two main effects. Firstly, as the Moon is either approaching or moving away from Earth during its orbit, the mutual gravitational pull of the Earth-Moon system accelerates or decelerates the Moon, respectively. This causes the Moon to move faster or slower than its average motion. Secondly, the Moon's motion appears faster in angular terms when it is closer to the Earth and slower when it is farther away.

The combination of these two effects results in a variation of the Moon's daily motion to the east, ranging from as little as 11.6 degrees per day near its apogee to as much as 14.8 degrees per day near its perigee.

The Moon's motion is constant relative to Earth and the Sun

The Moon moves at a mean orbital velocity of 1.022 km/s (0.635 miles/s, 2,286 miles/h) around the barycentre between the Earth and the Moon. This means the Moon covers a distance of about half a degree on the celestial sphere each hour. The Moon moves at a rate of about one whole Moon diameter (0.5 degrees) per hour, or 13.2 degrees per day. This means that the Moon moves a full cycle of 360 degrees in 27.3 days. This is the sidereal period of the Moon, a sidereal month. The Moon moves backwards in terms of the diurnal motion of the sky, that is, eastwards.

The Moon is in synchronous rotation, meaning that it keeps the same face toward Earth at all times. This happens because the Moon rotates at the same rate as its orbital motion, a special case of tidal locking. The Moon's rotational axis is inclined by a total of 6.7° relative to the normal to the plane of the ecliptic. This leads to a perspective effect in the north-south direction, referred to as optical libration in latitude, which allows one to see almost 7° of latitude beyond the pole on the far side of the Moon.

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