Supermoon surprise to light up night sky this week

A comparison of the December 2017 full moon at perigee (closest to Earth for the month) and a full moon in June 2017 at apogee (farthest from Earth for the month). Picture: Muzamir Mazlan at Telok Kemang Observatory, Port Dickson, Malaysia
A comparison of the December 2017 full moon at perigee (closest to Earth for the month) and a full moon in June 2017 at apogee (farthest from Earth for the month). Picture: Muzamir Mazlan at Telok Kemang Observatory, Port Dickson, Malaysia

Perigee-syzygy moon just rolls off the tongue doesn't it? Well, supermoon is a bit easier to say and it is happening this Wednesday, April 8.

The moon's orbit is not a perfect circle - it actually elliptical. That means that as the moon moves around the Earth in its elliptical journey, sometimes it is actually closer to the Earth and sometimes it is actually further away.

On average, the moon is 384,400 km away. In practical terms, if you were to drive 100km/hr, it would take you a little over five months of driving to reach it - a very long road trip. However, the orbit of the moon varies by about 50,000km.

Perigee is the point in the orbit of an object (in this case the moon) when it is closest to the object it orbits (in this case the Earth).

Syzygy (that really is a mouthful) refers to when three celestial bodies are in a line, such as the sun, Earth, and moon. You are more familiar with this occurrence than you think - it is what causes the full moon. When the moon is on the opposite side of the Earth to the sun - specifically when they are 180 degrees apart - we get a full moon. It is because of this that when a full moon rises in the eastern horizon, it happens right around the same time the sun sets in the western horizon. So if you see a moon rising about the same time of the setting sun, it will be a full moon.

On average, the moon is 384,400 km away. In practical terms, if you were to drive 100km/hr, it would take you a little over five months of driving to reach it - a very long road trip.

But have you noticed that the full moon always appears bigger on the horizon? You are not alone - people have for thousands of years. It is mostly our brains playing a trick on us and it is all because of perspective. The atmosphere does cause a bit more refracting of the moon near the horizon (the same cause as the red/orange sunset).

However if you hold out your thumb when the moon is on the horizon, and then a few hours later when it is higher, it will be about the same relative size. On the horizon, you also have objects in front of you - trees, houses, etc., that make it look bigger.

When the moon is high in the sky, you don't and so it looks smaller. So if the moon will be closer on Wednesday, will it look bigger? Unlikely. Between perigee-syzygy moon and apogee-syzygy moon (apogee is the furthest point in the orbit and so this is sometimes called a micromoon), the moon appears about 15 per cent different, so not a lot when even compared to the other extreme.

A comparison between the December 3, 2017 full moon at perigee (closest to Earth for the month) and 2017s farthest full moon in June at apogee (farthest from Earth for the month). Picture: Muzamir Mazlan at Telok Kemang Observatory, Port Dickson, Malaysia

A comparison between the December 3, 2017 full moon at perigee (closest to Earth for the month) and 2017s farthest full moon in June at apogee (farthest from Earth for the month). Picture: Muzamir Mazlan at Telok Kemang Observatory, Port Dickson, Malaysia

This perigee-syzygy moon is right before Easter - which is also no coincidence. Easter is the Sunday after the full moon after the northern vernal (spring) equinox. By definition, every lunar cycle will have a point where the moon is at perigee and apogee, meaning about one to two times per year we will get a perigee-syzygy moon. So not unsurprising.

So when you are outside on Wednesday night (keeping a safe distance away from others) and notice the beautiful, big, bright moon - it is not really that big or that special, but enjoy it regardless.

  • Brad Tucker is an astrophysicist and cosmologist at Mount Stromlo Observatory at the Australian National University.
This story Supermoon surprise to light up night sky this week first appeared on The Canberra Times.