A supermoon is the name given to a full moon when it’s at its closest point to Earth, also known as its perigee (pronounced PEH-ri-jee). This means that it’s less than 364,000 kilometres from us and so it looks significantly larger and brighter than at any other time. Although the moon is at its perigee once every 28 days, it’s only referred to as a supermoon when this coincides with a full moon. This happens between 4 and 6 times every year.
Tonight, for the first time since 1982, a lunar eclipse will coincide with a supermoon. A lunar eclipse occurs a couple of times a year on average (228 will occur in the 21st century alone) and happens when the Earth moves between the Sun and the Moon. This means that the Earth’s shadow gradually moves across the face of the moon. According to NASA a lunar eclipse only coincides with a supermoon every few decades; the last time it happened was 1982 and the next will be 2033. To make tonight’s moon an even more spectacular sight, it will appear to turn red. This happens when the Earth gets in the way of the Sun’s light; our atmosphere scatters the blue part of the spectrum (which is why our sky and large bodies of water appear to be blue) so the light that does fall onto the Moon’s surface is more red than usual. The exact colour of the moon may vary according to the amount of dust in our atmosphere, so we won’t know what it will look like until it happens.
The times for tonight’s eclipse are as follows (all times are in BST: if you’re not in the United Kingdom you may need to check the time difference and adjust accordingly):
01:11 The Moon touches the edge of the Earth’s shadow, known as the penumbra
01:50 The Earth’s penumbra begins to be visible on the Moon’s left side, looking like subtle shading
02:07 The Moon passes deeper into the Earth’s central shadow, called the umbra. As it moves deeper into this shadow the moon will become darker in colour, taking on a red, orange or brown hue.
03:11 The total eclipse begins; the Moon is completely within the Earth’s umbra.
03:47 The Moon is in the centre of the Earth’s shadow and will be at its most striking
04:23 The Moon begins to pass out of the Earth’s umbra, and will gradually brighten until it is completely clear of the Earth’s shadow at 06:22.