The 21 March 2019 Supermoon, or the more common name, Perigee Syzygy of the Sun, Earth and Moon system.

Most models of the Moon orbiting the Earth depict its orbit as being circular but we know from Kepler’s Laws that celestial bodies orbit one another in an ellipse rather than a circle. What this means is that sometimes the Moon is closer to the Earth than at other times in its orbit. When this is combined with the Moon being a Full Moon then it’s commonly known as a Supermoon. The last of these occurrences for this year is tonight, the 21 March 2019. At 8:30pm the Moon was 360,242 km away, about the distance it takes light to travel in 1.2 seconds. This is the third Supermoon this year, so basically all of the full Moons this year have been Supermoons.

By comparison the full Moon in August last year was 399,892 km away (1.33 light seconds) and its diameter appeared to be 29.9 arc minutes. Compare that to tonight where the apparent diameter of the Moon is 33.2 arc minutes so a little over 10% bigger than when the Moon is the farthest away.

Now an important thing to remember is that we did not actually see the full Moon at its closest approach because the two occurrences don’t actually line up that well. The Moon was full a few hours before it rose in Wellington, so technically we missed the Full Moon. But at 1am in the morning we will see the Moon at its closest approach where it will be 355,590 km (1.19 light seconds) and 33.6 arc minutes in apparent diameter. At 1am the Moon will be 99.3% illuminated so it will certainly appear to be full but in reality it’s just past being at maximum illumination. So there’s no need to be picky but technically it’s not a Supermoon, then again the term Supermoon has no real scientific basis anyway other than to describe the time when the Moon is full and close to perigee. Out of interest the technical name is perigee syzygy. The term syzygy refers to the straight line of three bodies, in this case the Moon, the Earth and the Sun all lined up.

When observing the Moon through a telescope the best time is when the Moon is not fully illuminated. This is because the Sun then casts shadows over the Moon’s craters and they stand out. In the picture above the Moon was about 90% illuminated and clearly shows some of the spectacular views you can get of the Moon near the terminator. The terminator is the name of the line that separates shadow from light on the face of the Moon.