A lunar eclipse occurs when the Moon passes through the Earth’s shadow and it happens reasonably frequently so that we get to enjoy the spectacle. It is sometimes called the Blood Moon. This is because the Moon goes red – or more of an orange colour than pure red. Photographers produce stunning images which show the red and the lunar surface detail. The final lunar eclipse for 2021 occurred with the Moon positioned just above Pleiades and a few photographers were able to catch both the eclipse and the beautiful star cluster in the same shot.
Hari and I have mentioned a few times we are not big fans of the Moon, though acknowledge its importance for the continued stability of the planet. A lunar eclipse offers the perfect opportunity to observe galaxies during a Full Moon. So it made perfect sense that we decided that the 16″ reflector would be the choice of telescope for observing the lunar eclipse. Many others would have opted for a smaller more portable telescopes – but then they probably weren’t going to look at the Grus Quartet of galaxies.
We no longer do our stargazing from Stonehenge Aotearoa, mainly due to the significant challenges of operating there but also because of the light pollution from both the nearby towns and the cars on roads that are quite close to the site. Our new site is truely stunning, nestled in the Eastern hills of the Wairarapa, the site is elevated and well away from roads and other sources of light pollution. As this is going to be the new site for Star Safari the Lunar Eclipse offered the perfect chance to test it out. We had some friends come along as well so it ended up being a bit of a star party.
The weather was fantastic though with a little wind to keep everyone cool. The seeing conditions improved throughout the evening, which is typical of this time of the year as the atmosphere stays a little unsettled from the heat of the day. Earlier in the week the weather had been terrible with very high winds so at least they had gone!
The Moon appeared above the horizon at about 8:30pm and you could already see a chunk of it missing. The photo below shows what it looked like through the telescope, via my iPhone. The orientation in the sky was the chunk out of the Moon was on the bottom, as the telescope is a reflector the image is rotated 180 degrees and I wasn’t holding the phone straight – hence the funny orientation to what people may have observed.
And then we just waited. It was good to have a look at Saturn and Jupiter though the seeing was a bit rough so the planets weren’t great. As the evening got darker and more of the Moon started to be in the Earth’s shadow a red hue became visible on the darkened lim. This eclipse was a partial eclipse meaning the Moon didn’t quite get all of the way into the Earth’s shadow so there was a very bright bit remaining, though it was quite small and the red effect was certainly there. As the Moon got further and further into the Earth’s shadow other features of the night sky also started to appear, such as the Magellanic Clouds and the Milky Way. Normally in a Full Moon these would mostly disappear as the Moon is so bright.
When the Moon mostly got into the Earth’s shadow it was a great opportunity to check out some of the galaxies at our new observing site, that’s why we have a 16″ telescope! The first ones we looked at were in the constellation of Grus, known as the Grus Quartet. These four galaxies are fantastic to view with three of them close together and one a bit further away. These galaxies are 60 to 80 million light years away and through the big telescope they are awesome. The next group of galaxies we had a look at were the Fornax cluster, which are a similar distance to the Grus Quartet. The Fornax cluster is fantastic to look at as you can get about 11 galaxies in the eyepiece all at the same time, with the right eyepiece of course. It was amazing to see these galaxies during a Full Moon, which would normally be impossible. A good test of the new site was to see if we could also observe the famous Northern sky object, Andromeda Galaxy (M31). At the old site this was always firmly in the light pollution of Masterton but the extra elevation and better distance from Masterton meant at the new site the galaxy was clearly visible. We could also even see the Triangulum galaxy (M33).
Now for some celestial mechanics: the Moon orbits the Earth and that orbit is slightly elliptical. All orbits are elliptical, Johannes Kepler figured that out in the 1600s. There’s about a 12% difference in the perigee and apogee of the orbits, the apogee is the part of the orbit farthest from the Earth. During the eclipse this distance was 403,114km, at perigee it’s around 360,000km distant. Being a bit further away in its orbit it goes a bit slower. At perigee the Moon is humming along at about 1.09 km per second and at apogee it’s a bit slower at about 0.97 km per second. So at apogee the Moon takes a bit longer to go through the shadow of the Earth – hence why this eclipse was the longest for about 600 years. The Moon goes red because of the scattering of the Sun’s light as it goes through the Earth’s atmosphere. Red light is a longer wavelength so gets scattered less than the blue light, meaning is mostly red light that ends up illuminating the Moon when it’s in the Earth’s shadow.
The next Lunar Eclipse is about this time next year.