Image: CC0 Creative Commons
The longest total lunar eclipse of the 21st century will occur on 27 July 2018, and it will be visible to all photographers in the UK – clear skies allowing.
The spectacle will see the moon turn an orangey, reddish colour for over an hour, which can make for a unique photograph.
A total lunar eclipse is not to be confused with a total solar eclipse, a much more dramatic and short-lived event that occurred last year in the US. While a challenging subject to photograph, a total lunar eclipse as far easier to capture using any camera equipped with a decent zoom capability.
Here's everything you need to know to capture the most dramatic 'blood moon' of your lifetime.
What is a total lunar eclipse?
A total lunar eclipse occurs when the moon moves into the darkest part of the Earth's shadow projected into space. It can only occur during a full moon, where Earth is precisely between the sun and our satellite.
Although a full moon occurs every 29 days, the moon has an inclined and rather wobbly orbit around Earth, which means it very rarely passes through the Earth's shadow. When it does pass through some of the shadow, a partial lunar eclipse is the result.
All that happens during a partial lunar eclipse is that the full moon is much duller (and therefore a lot easier to photograph than usual). That's also how a total lunar eclipse begins and ends, but only during a total lunar eclipse does the moon enter the darkest part of the Earth's shadow, in which it turns red.
Why does the moon go red?
As it begins to enter the dark shadow of Earth, one edge of the moon begins to look pinky-orangey-brown, changing to red as the event peaks. Once it has entered into that darkest zone of the shadow, it’s what's called totality, where sunlight is being bent through the Earth’s atmosphere and on to the moon.
Read more: How and when to photograph the moon
So, much like a sunset is red because the light that’s entering your eyes is travelling through a lot of the Earth's atmosphere, so the light projected onto the lunar surface is also red. The physics is much the same.
Do I need special equipment?
Unlike a total solar eclipse, for which special glasses (for humans) and ND filters (for cameras) are essential for viewing the partial phases, the entire spectacle of a total lunar eclipse is very easy on the eyes. No special equipment is needed, just a zoom lens or a telescope.
Where is the blood moon visible from?
Unlike a total solar eclipse, which is visible from only a very narrow track upon the Earth’s surface – and only then for a few minutes – a total lunar eclipse is more egalitarian. In fact, the entire night side of Earth can see a total lunar eclipse.
The total lunar eclipse that will occur on 27 July 2018 will be visible form the UK, Europe, Africa and Asia. So will the UK get a good view? That depends on what you want from it.
From an astronomical perspective, it's best if the moon enters the Earth's shadow while it’s high in the sky from where you are observing from. That way, you get to see the whole event, including the partial phase leading up to totality, totality itself, and the following partial phase. But that’s not what’s going to happen on 27 July 2018.
Exact timings for moonrise, sunset and totality
The moon will rise at 20:49pm and the sun will set at 20:56pm (in London), and by 21:21pm the moon will be at its maximum eclipse, though it won't stop being blood red until about 22:13pm, when totality will cease. UK observers will then see a partially eclipsed moon until 23:19pm, when the full moon will gradually return to its regular brightness as it exits the Earth's shadow. Either way, UK photographers have a great chance – and a lot of time – to get a special shot.
What time will the blood moon be visible from the UK?
In the UK only totality and the second partial phase will be visible. In fact, as the moon rises above the eastern horizon at 20:49, it will already be in the totality phase.
In effect, UK observers will have missed the first 20 minutes or so of totality. The reason why that is a positive for photographers is that as the moon rises above the horizon, it will be blood red. Given clear observing conditions, that's going to look really, really strange.
Read more: How to create a moonstack
It's also going to suppress the brightness of the moon quite substantially, so it should be easy enough to photograph within an interesting composition. It's quite rare to have the chance to photograph a blood moon within a landscape; usually it's photographed high in the sky against a black background.
How to photograph a total lunar eclipse
Given clear skies – which certainly isn't inevitable in the UK, especially close to the horizon – photographing a total lunar eclipse is not technically difficult.
The first thing you need is a clear eastern horizon so you can maximise your chances of seeing the 'bloody' moonrise if the skies are clear. You can use apps like PhotoPills and The Photographer's Ephemeris (TPE) to precisely plan your shot; try to get the moon rising between buildings or trees, and consider finding a pond or lake for reflections.
While any camera with a wide-angle lens can be used to capture the landscape, the redness of the moon will be better captured by using a zoom lens, preferably one over 200mm (and ideally 400mm or more).
That your camera must be placed on a tripod is obvious, but there are a few other things to think about that are unique to this total lunar eclipse. Most online tutorials are tailored to photographing lunar totality when it's against a dark backdrop, but this one will be during the blue hour and within a landscape.
Shutter speed is key factor. Since the the moon orbits Earth at a speed of 2,288mph and the Earth is rotating at about 600mph at the UK – it rotates faster at the surface the closer you are to the equator – the biggest problem is avoiding a blurry image. That means you must always use a high shutter speed to get sufficient sharpness.
To avoid blur, never use a shutter speed slower than 1/100sec. The sensitivity can stay within ISO 200 to 800, though by 22:00pm it’s still going to be red, and by that time, the sky will be dark, so you could push your ISO further.
Read more: How to photograph during the blue hour
While the aperture will need to be narrower than usual for a lunar totality at 21:00pm, by 22:00pm it should be wide open at f/4 (or f/2.8 if your lens can cope with that). It’s hard to be precise since the brightness of the totally eclipsed moon is a mystery until it appears, so just keep the camera in its Manual exposure mode and experiment with various apertures and ISO settings.
When is the next total lunar eclipse?
The next total lunar eclipse visible from the UK will be on 21 January 2019, though that one begins during the middle of the night at 04:41am and will lasts just 62 minutes (as opposed to the 1hr 43mins that will occur on 27 July 2018). On average a total lunar eclipse occurs every three or four years from one location, although they can occasionally be more common.