Friday, 18 January 2019

Perigee Total Lunar Eclipse - Monday 21st January 2019

Perigee Total Lunar Eclipse - Monday 21st January 2019

Overnight on Sunday 20th/Monday 21st January in the UK, we are in for a real treat in the form of a total lunar eclipse. You will no doubt have seen the press reports about this “Super-Wolf-Blood-Moon” and are probably wondering what the heck that is.

The term “supermoon” is not one that is favoured by astronomers. It is a term coined by an astrologer a few years ago and in recent years has been used extensively in press reports. To understand what “supermoon” is referring to you need to understand the lunar orbit. The path the Moon takes around the Earth is not a perfect circle; it is in fact an oval shape. So that means that once a year the Moon is slightly closer to Earth, and once a year it is further away. The point when the Moon is at its closest is called “Perigee” and this is the term astronomers prefer to use. The point when it’s furthest away from Earth is called “Apogee” (the astrologers call this a “minimoon”). When the Moon is at Perigee, it is just under 13% larger in diameter and about 30% brighter. 

The above photograph shows two lunar eclipses which took place in 2018, one during a Perigee Moon the other during an Apogee Moon. At a glance, the difference looks impressive, but given how small the Moon actually is to the naked eye, you will struggle to notice any difference unless you are looking at it through a moderately sized telescope. The photo below is a panoramic photograph I took showing the eastern sky around sunset, with a rising Gibbous Moon in the sky. As you can see, the Moon is really tiny compared to the background. If you can see a 13% difference in diameter at this scale, then you definitely have better eyes than I have! To see my full blog titled "How Super is a Supermoon?" click here

The eclipse which is taking place on Monday morning is during a Perigee Full Moon, so that’s quite special.

So where does the “wolf” part come from? Native Americans used to name all of the Full Moons in a calendar year and January’s Full Moon is called the “Wolf Moon”. And the “Blood Moon” bit comes from the fact that the Moon turns a shade of red during a total lunar eclipse, and in the USA this is often referred to as a “Blood Moon”.

During a Lunar Eclipse, the Earth passes between the Sun and the Moon, and the Earth is casting a shadow into space. When the Moon passes through this shadow, it causes a lunar eclipse. This can only happen during a Full Moon, but because the Moon’s orbit is slightly tilted, the alignment is only correct to cause a total eclipse approximately once a year. 

The shadow cast by Earth has two regions; the penumbra and umbra. When the Moon passes into the penumbra, the Moon’s light will dim slightly. But when it passes into the umbra, a visible dark shadow will begin to pass across the surface of the Moon. Once the Moon is fully in shadow, it will look a shade between brown and red. The exact colour cannot be predicted because it depends on atmospheric conditions such as dust, pollution and humidity levels from your location.  The totality period of a lunar eclipse lasts around an hour. This is in stark contrast to a total eclipse of the Sun, which only lasts a couple of minutes! This is because the Earth is bigger than the Moon and the shadow cast is quite large. This means it takes a few hours for the Moon to pass completely through it.

As mentioned earlier, lunar eclipses happen approximately once a year. But a total eclipse during a Perigee Moon is much less common. The last one we had was back in September 2015. The photos below are the ones I took during that eclipse (to say had some technical issues during this eclipse would be understating things!). The next one will be on 26th May 2021, so a Perigee Total Eclipse is definitely a special event.

Unfortunately, if you want to observe this eclipse, you are going to need to either stay up all night, or set a very early alarm. The event begins at 02:37 GMT on Monday 21st January. The Totality phase begins at 04:45 GMT and will last for about an hour. At this time the Moon will over in the south western sky. The full timings are the table below and are in GMT (which is the same as UT):

02:37  Moon enters penumbral shadow and will begin to look dimmer
03:30  Moon enters umbral shadow. Shadow will begin to spread across the Moon from the left hand side
04:45  Total Eclipse begins. The Moon will look redish-brown
05:45  Total Eclipse ends. The top left hand side of the Moon will begin to emerge from shadow
06:53  Moon emerges from umbral shadow. Dawn will be breaking
08:14  Moon sets

And this is a simulated view of what the Moon will look like during these different phases:

Most people know that I’m not a big fan of gimmicky headlines and sensationalist reporting when it comes to astronomical events. I’m not a killjoy; I just think that the universe is amazing enough without it. But anything that gets people out and looking up is a good thing, so I really hope that you do make the effort to get up at that ungodly hour to observe this amazing event. I don’t need a “Super-Wolf-Blood-Moon” to stay up all night; a Perigee Lunar Eclipse is good enough for me!

Thanks to Stuart Atkinson and Petra Horalek for allowing me to use their graphics in this blog.