Moon's Daily Trek: East To West

The Moon is a constant presence in our night sky, but its appearance is ever-changing. The Moon orbits the Earth once every 27.3 days, and because it spins on its axis at the same rate, the same side is always facing us. This is known as synchronous rotation. The Moon's orbit is elliptical, and it travels 12-13 degrees east every day, causing it to rise about 50 minutes later each day. The Moon's orbit is also tilted about 5 degrees compared to the Earth's orbit around the Sun, which prevents frequent solar and lunar eclipses. The Moon's orbit around the Earth, along with the Earth's spin and orbit around the Sun, contribute to the Moon's ever-changing appearance, or phases, which repeat every 29.5 days.

The Moon's elliptical orbit

The Moon's orbit around the Earth is elliptical, not perfectly circular. This elliptical orbit is also referred to as an elongated circle. The Moon's orbit has a mean eccentricity of 0.0549. This means that the Moon's distance from the Earth's centre varies, with a mean distance of about 385,000 km (239,000 mi) from the Earth's centre. The Moon's orbit has a semi-major axis of 384,400 km and a semi-minor axis of 383,800 km, which is a difference of only 0.16%.

The Moon's orbit is also inclined by about 5.1 degrees with respect to the ecliptic plane, whereas the Earth's equatorial plane is tilted by about 23 degrees with respect to the ecliptic plane. The Moon's orbit is tilted by about 5 degrees compared to the plane of the Earth's orbit around the Sun. This tilt in the Moon's orbit means that it usually passes above or below the Sun when it passes between the Earth and the Sun. This tilt also prevents monthly solar and lunar eclipses from occurring.

The Moon's orbit around the Earth is constantly changing due to the gravitational attraction of the Sun and other planets. The Sun's gravitational effect on the Moon is more than twice that of the Earth's on the Moon. The Moon's orbital velocity is also slower at apogee, and it covers a greater orbital distance to reach New Moon near apogee compared to perigee.

The Moon's rotation

The Moon takes about 27.3 days to orbit the Earth, and during this time, it also spins on its axis exactly once. This means that its day is as long as its year. The Moon's orbit is not a perfect circle but is slightly elliptical, with an eccentricity of 0.05. This means that it is slightly closer to the Earth at one point in its orbit than at another. The average distance between the Earth and the Moon is 384,400 km (238,855 miles), but this varies between 363,104 km and 405,696 km due to the elliptical orbit.

The synchronous rotation of the Moon has some interesting effects on our perception of it. For instance, the Moon appears to go through different phases of illumination, which are caused by the changing positions of the Earth, Moon, and Sun. The phases of the Moon repeat every 29.5 days, but its orbit around the Earth takes only 27.3 days. This difference is because, during the time the Moon orbits the Earth, the Earth also moves around the Sun, causing the Moon to take a little longer to return to the same position relative to the Sun.

The Moon's distance from Earth

The Moon is, on average, 238,855 miles (384,400 km) away from Earth. This is about 30 Earths away. The Moon's orbit is elliptical, not a perfect circle, so its distance from Earth varies. When the Moon is at its furthest point from Earth, it is about 405,696 km (252,088 miles) away, or almost 32 Earths. At its closest, the Moon is 363,104 km (225,623 miles) from Earth, or between 28 and 29 Earths.

The Moon's orbit is not centred on Earth, but rather, Earth is located at one of the foci of the Moon's elliptical orbit, meaning it is closer to one edge of the orbit than the other. The eccentricity of the Moon's orbit is 0.05, which is relatively close to a perfect circle. For comparison, Mercury has the most eccentric orbit of all the planets in our Solar System, with a value of 0.2, while Venus's orbit is the least eccentric, with a value of 0.007.

The Moon is currently moving away from Earth at a rate of approximately 3.8 cm per year, which is roughly the same rate at which human fingernails grow. In the far future, the Moon will appear smaller in the sky, and total solar eclipses will no longer occur as the Moon will not be large enough to completely obscure the Sun.

The Moon's phases

The Moon is always half-lit by the Sun (except during a lunar eclipse). The side of the Moon facing the Sun appears bright because of reflected sunlight, and the side of the Moon facing away from the Sun is dark. Our perspective on the half-lit Moon changes as the Moon orbits Earth. When the side nearest to us is fully lit, we call this a full moon. When the far side is fully lit and the near side is dark, we call this a new moon. When we see other phases, we are looking at the division between lunar night (the dark part) and day (the bright part).

The four primary Moon phases occur about a week apart, with the full moon being the most dazzling stage. The Moon is said to be waxing when it is becoming more illuminated each day, and waning when the light is decreasing. The phases between the primary phases can be understood by breaking down four key words: crescent, gibbous, waxing, and waning. Crescent refers to the phases where the Moon is less than half illuminated, while gibbous refers to phases where the Moon is more than half illuminated. Waxing means "growing" or expanding in illumination, and waning means "shrinking" or decreasing in illumination.

Thus, after a new moon, the Moon is in the waxing crescent phase, followed by the first quarter, then waxing gibbous, and finally the full moon. After the full moon, the Moon is in the waning gibbous phase, followed by the third or last quarter, and then the waning crescent phase before it returns to a new moon.

The Moon's influence on Earth

Secondly, the Moon is responsible for the tides on Earth. The Moon's gravitational force creates a "bulge" in the oceans, resulting in high tides. Simultaneously, on the side of the Earth opposite the Moon, the gravitational pull is weaker, creating another high tide. As a result, we have two high tides and two low tides every day. The Moon's influence on tides is significant due to its proximity to Earth. While the Sun also contributes to tides, its effect is only about a third of the Moon's influence.

The Moon also has a significant impact on the length of our days. Early Earth spun much faster, resulting in shorter days. Over time, the Moon's gravitational pull has stolen some of the Earth's spin energy, causing the Moon to move to higher orbits and slowing down the Earth's rotation. Evidence suggests that billions of years ago, Earth had a six-hour day, and the length of the day has gradually increased due to the Moon's influence.

Additionally, the Moon's presence has influenced the evolution of life on Earth. The Moon's light, reflecting the Sun's, provides illumination for humans and animals at night. This added nighttime illumination has been documented to impact the success rates of predators and the foraging patterns of prey animals. Furthermore, certain species of corals, crabs, worms, and fish rely on specific phases of the Moon to trigger reproduction. The Moon's cycles have also guided humanity for millennia, with calendar months roughly aligning with the time between full moons.

Lastly, the Moon's gravitational pull has affected the Earth's geology. Billions of years ago, when the Moon was much closer to Earth, extreme tides occurred every three hours due to the Earth's rapid spin. These tides eroded coastal areas, adding essential minerals to the oceans, which may have played a vital role in the rapid evolution of life.

Frequently asked questions