The moon is our closest neighbour in the cosmos and it has an ancient and enthralling history with the earth. Unlike our sun, the moon pops up to see us during both night and day, with an ever-changing appearance. In this short blog post I want to discuss some of my favourite facts about our cratered cosmic companion.

Origin:

The Moon is old, like, really old. At 4.527 ± 0.010 billion years old, the Moon is almost exactly the same age as planet Earth (Earth is c. 4.54 billion years old). This is almost a third of the time that the universe has even existed (the best estimate for the age of the universe is 13.798±0.037 billion years old). Although the origin of the moon is still academically discussed and debated, the prevailing hypothesis, which is supported by a wide body of evidence, is that the moon was the by-product of a colossal cosmic collision. Here, a Mars-sized object smashed into the early planet Earth, ejecting a gigantic amount of rock and dust into space. Over time, gravitational interactions caused these orbiting objects to coalesce and form an early moon. This collision would have been epic on all proportions; trillions of tonnes of material would have been instantly vaporised and temperatures on Earth would have risen above 10,000°C. An artistic impression of this impressive event is shown below.

Since the Moon’s formation, it has been receding from planet Earth at a rate of 378 mm per year (on average). This means that when bacteria first evolved on the planet (c. 3.75 billion years ago) the Moon would have been around 30,000 km away from the planet, compared to the 384,400 km distance we see today. The Moon that the first bacteria and life on Earth never got to see would have appeared gigantic in the sky! Even the Moon that the dinosaurs never appreciated would have appeared much, much larger than our own.

Tidal locking and the Moon’s orbit:

Shortly after the moons formation, the moon became tidally locked with the Earth. What does this mean? Put simply, it occurs when gravity makes two orbiting bodies always face each other. Have you ever wondered why the moon always appears to show us the same face, even though it rotates on its axis? Well, this is the reason; the moon is tidally locked to the Earth and spins on its axis at almost the same speed as it orbits the Earth. This is shown clearly in the GIF below.

I learnt something interesting about the Moon’s orbit just last week. If you were to imagine drawing the orbit of the moon around the Sun, what would it look like? If you think along the same lines as me, you may have drawn a similar orbit to the one below:

However, this is not the case. Counter intuitively, this is not how the Moon’s orbit around the Sun appears. Look at the diagram below. The Moon’s orbit around the Sun is a near perfect circle (technically an ellipse). This completely surprised me and this took me awhile to get my head around, but the more you think about it, the more it makes sense. A simple way to visualise this is to imagine two racing cars driving around a circular race track (you are driving one of them). One of the cars overtakes you and pulls into the lane in front of you. So, being annoyed by this you overtake him and pull in front of his car. This pattern repeats itself many times. From the driver’s perspective, it appears like you are doing circles around each other. However, when seen from above, both of the cars are moving forward the whole time, and consequently will appear to have driven around in a circle (or more accurately an ellipse).

It is, of course, the Moon’s orbit around planet Earth that causes the Moon to go through its monthly cycle, from new moon to full moon. The diagram below shows this nicely, as well as demonstrating how the Moon’s orbit around the Sun is indeed elliptical.

Size and distance to Earth:

Although our Moon is only the 5th largest moon in the solar system, its relative size compared to its orbiting planet (i.e., planet Earth) is the largest. Relative to the size of our planet, the moon really is huge; it is ¼ of the diameter of Earth and 1/81 Earth’s mass. The Moon orbits at an average distance of 384,400 km. Although a mere nanometre relative to the size of the cosmos, this is actually pretty far away. If you wanted to drive to the moon in your magic flying car (travelling at 100 km/h) it would take 3844 hours, or 160 days. Flying on a jumbo jet (travelling at 900 km/h), it would take 427 hours, or nearly 18 days; and you thought it took a long time to fly to New Zealand! The picture shown below shows the distance between the Moon and the Earth to scale. Here, each pixel represents 500 km.

This distance is so large that all seven planets in our solar system could comfortably fit between the Earth and the Moon…with 8030 km spare, so if you want to include Pluto in this, it would easily fit, too! This is another thing I only realised a few days ago, and it doesn’t really seem to make sense; the moon appears so close to us in the sky. The numbers tell us otherwise. The picture below shows the Earth (far left), the planets and the distance to the Moon (far right) to scale.

The Moon and life on Earth:

It is no exaggeration that life, as we know it today, would not flourish on Earth if it was not for the Moon. The formation of the Moon was likely one of the most important events that lead to us being here today. I know what you may be thinking: what has the Moon possibly done for us? It’s just a spherical rock, right? Firstly, before the Moon’s formation c. 4.5 billion years ago, a day on planet Earth lasted just 5 hours! Over the eons, the gravitational interactions between the Earth and the Moon slowed Earth’s rotation on its axis. Consequently, the days increased in length. This inevitably changed the course of biological evolution on Earth. However, this is not the main reason we owe a lot to the Moon. All planets wobble slightly on their axis. If it were not for the our Moon, Earth would erratically and stochastically wobble on its axis. The result: Earth would literally change seasons on a daily or weekly basis. For life as we know it, this would be a very bad thing. The Moon’s stabilising gravitation interactions ensure the Earth remains on a relatively stable axis of rotation. So, next time you see the Moon, may it be day or night, look up to our cosmic companion and tip your hat; we owe a lot to that beautiful, cold, distant space rock. And always remember, we have walked on the surface and even driven a badass buggy across its rocky terrain.

I will leave you today with the brilliant mind of Eddie Izzard:

“When the Americans landed on the moon, that was the point when God should have come up and said hello. Because if you invent some creatures and you put them on the blue one and they make it to the grey one, then you f#cking turn up and say, ‘Well done.’ It’s just a polite thing to do.”

Eddie Izzard

Moon Gallery:

Here are some pictures captured by a fellow Winslow boy, Keith Bidey, using just a digital SLR camera (Pentax K10D with a 150-500mm sigma lens).