Lunar eclipses occur when our closest celestial neighbour in space, the Moon passes through the Earth’s shadow and can only occur at a full moon. So why don’t we see a lunar eclipse once a month at every full moon? The Moon’s orbit is tilted by 5 degrees to the Earth’s orbit around the Sun, so the Moon actually spends most of its time above or below the plane of Earth’s orbit. Thus, most full moons do not pass through the Earth’s shadow and no eclipse occurs. However, about two to four times a year some part of the Moon does pass through the shadow and an eclipse occurs.
The Earth’s shadow consists of two parts – the penumbra and the umbra. The penumbra is the outer part of the shadow and is where the Earth blocks part but not all of the Sun’s direct light from reaching the Moon. The umbra or inner part of the shadow is the area where Earth blocks all of the Sun’s direct light from reaching the Moon. About 35% of lunar eclipses are total eclipses and during this type of eclipse, the entire Moon passes through the umbra part of the shadow.
The Moon is still illuminated in the umbra due to indirect sunlight that has been refracted in, and emerges from, the Earth’s atmosphere. The dust and air in Earth’s atmosphere scatters the blue coloured light and allows red light to pass by. The remaining light is a deep red or coppery colour. So, if the air is clear we see a coppery coloured Moon. However if there is extra dust etc. in the atmosphere (maybe due to a volcanic eruption) the eclipsed Moon can turn darker shades of red, sometimes even completely black as was the case on October 4th, 1884. This was due to the eruption of Krakatoa. It is also interesting to consider that if the Earth had no atmosphere then the Moon would be completely black during totality.
Unlike solar eclipses, lunar eclipses are safe to watch with your eyes, you don’t need any special protective filters or even binoculars or a telescope, though these do help make the coppery colour stand out.
Total Lunar Eclipse on 28 August 2007
Amateur astronomers can make some very useful observations during total eclipses. A simple one is to use the Danjon Brightness Scale to determine the Moon’s colour and brightness during totality.
Timings for the eclipse on 26 May 2021 in Australia and New Zealand are listed in the table below.
Observers Australia and New Zealand will see the entire eclipse.
Eclipse EST WST NZST UT
(26 May) (26 May) (26/27 May) (26 May)
Penumbral Begins 6:47pm 4:47pm 8:47pm 08:47
Partial Begins 7:45pm 5:45pm 9:45pm 09:45
Totality Begins 9:10pm 7:10pm 11:10pm 11:10
Mid-Eclipse 9:19pm 7:19pm 11:19pm 11:19
Totality Ends 9:28pm 7:28pm 11:28pm 11:28
Partial Ends 10:53pm 8:53pm 12:53am* 12:53
Penumbral Ends 11:49pm 9:49pm 01:49am* 13:49
* 27th May.
The next total lunar eclipse visible in Australia (New Zealand be on 8 November 2022 so not too long to wait!
There will be a partial eclipse later this year on 19 Nov 2021 which is almost total but not quite – a small sliver of the Moon misses the Earth’s umbra shadow.
It is difficult to predict the colour of totality in advance and this is perhaps one of the most magical aspects of total eclipses of the Moon. Whether this is your first eclipse or one of many, this unpredictability makes each and every eclipse exciting and definitely worth a look. Make sure you don’t miss out on your chance to view something a little different this month!
Dawes G., Northfield P., Wallace K. (2020). Astronomy 2021 Australia, Quasar Publishing.
Lomb, N. (2020). 2021 Australasian sky guide. Ultimo, NSW: Museum of Applied Arts and Sciences Media.
NASA (2013). Lunar Eclipses: 2021 - 2030. NASA. Accessed 24 April 2021, https://eclipse.gsfc.nasa.gov/LEdecade/LEdecade2021.html.
Espenak, F. (2020). Danjon Scale of Lunar Eclipse Brightness. EclipseWise. Accessed 30 April 2021, http://eclipsewise.com/lunar/LEhelp/LEdanjon.html.