Recently I had the privilege of taking a moment to myself while out in the bush and appreciated an often overlooked yet incredible (and necessary!) natural phenomenon. A simple moon rise.

Sitting there with a dramatic moonrise to the east contrasted by an intense, radiant sunset to the west, I felt caught between two cosmic forces. And that got me thinking…

Have you been out on a game drive and noticed that the full moon looks huge as it rises? Well, no it has not got bigger because you are on a game drive. It just appears bigger and brighter as a result of a combination of three factors.

The night sky has fascinated humankind for millennia. It instils a sense of wonder in our curious minds and provides us all with a great deal of perspective as to how small our entire human existence really is in the greater scheme of things.

Firstly, the phase in which the moon is in determines how much of the moon is visible to earth (not the actual size of the moon, that doesn’t change!)

Moon phases by NASA.

Phases

As the moon orbits the earth, it rotates at the perfect speed that only one side of it will always face the earth, known as the near side. In conjunction with this, we view the moon in a different state (of illumination) each night as it slowly waxes and wanes from new moon to full moon and back again. For the moon to complete a whole cycle and return to the exact same phase takes roughly 29.5 days and is known as a lunar month.

While we use the Gregorian calendar for a sense of time-keeping. The Gregorian calendar is a solar dating system used by most of the world where the months are slightly longer. The roots of this measure of timekeeping were initially established by ancient civilizations. With a lunar month being marginally shorter than our Gregorian month, we witness a ‘blue moon’ roughly every two and a half years where there are two full moons in a calendar month (and no, the moon doesn’t actually turn blue!).

A Bigger Moon

But what actually causes the moon to appear bigger is a combination of two factors:

Orbit

The Moon’s orbit is not circular, but rather it orbits the Earth in an elliptical orbit, meaning that it is not always the same distance from the Earth. The closest the point the Moon ever gets to Earth (called perigee) is 364,000km, and the farthest it ever gets (Apogee) is around 406,000km.

At Perigee the moon is 364,000km from the earth and at apogee, the moon is 406,000km away.

So the percentage difference in distance between the average perigee and the average apogee is roughly 10%. That is, if the Full Moon occurs at perigee it can be up to 10% closer (and therefore larger) than if it occurred at apogee. This is quite a significant difference, and so it is worth pointing out that the moon does in fact vary in its apparent size at different times throughout the year.

But that’s NOT what causes the moon to look huge on the horizon. A 10% difference in size cannot account for the fact that people describe the moon as “huge” when they see it low on the horizon. What’s really causing the Moon to look huge on such occasions is the circuitry in your brain. It’s an optical illusion, technically known as the Ebbinghaus Illusion.

Ebbinghaus Illusion

We know that the moon doesn’t physically change size over the course of an evening, so then why does our brain think it has?
This phenomenon is known as the Ebbinghaus Illusion. When the Moon rises, it is visible just over the horizon, and naturally, the human eye will compare the size of the moon to other visible objects (like trees, rocky outcrops and other familiar beings). The size of the moon relative to these is considerably larger (even at such a distance from earth). However, when the moon rises higher in the night sky, the human eye now has no other objects against which to compare the moon’s size and it then appears comparatively smaller.

In this image, the two orange circles are exactly the same size, but we perceive them as being different depending on what they are surrounded by, which demonstrates the Ebbinghaus Illusion.

So actually it is just our mind playing tricks on us, making us think that the size of the full moon changes during the course of an evening!

With the moon being an ever-present object in the night sky, it is quite easy to overlook. Don’t let that stop you, and on your next game drive, ask your ranger about the phase and size of the moon. Not only can it be quite humbling in giving some perspective as to how small we really are, but it will also give you an appreciation for the complexities of what is often overlooked.