An eclipse is an astronomical phenomenon that occurs when something obscures something else – it eclipses it! This may sound very simple, but there are various elements playing a part to cause an eclipse to occur. There are the different orbits, Moon phases, types of eclipses and distances from Earth to consider. Here, we will give you the very basics of eclipses.
Eclipses have been accurately predicted since the early stages of mankind’s history. Astronomers around 2300 BCE were predicting their occurrence, based on the relationships between the relative positions of Earth, the Sun and the Moon. More detailed knowledge of eclipses emerged during the golden age of Greek astronomy. Nowadays, we know almost everything there is to know about eclipses and can predict when they will happen thousands of years into the future.
Spectacular solar viewing
The major type of eclipse is called a solar eclipse. On Earth, the Sun is eclipsed, or obscured, by our Moon when it is in a new moon phase (brush up on your moon phases in this this article). Other planets also have solar eclipses, but ours are unique, especially when we witness a total solar eclipse.
When we look up from our perspective on Earth, the Moon is just the right distance and size from us to cover the circle of the Sun. This is also referred to as the Moon at perigee, which is when the Moon appears larger as it is the closest to Earth that it gets. When it moves in front of the Sun, the Moon’s disc appears to cover the entire disc of the Sun, and it will seem like night-time in the middle of the day.
In comparison, if you were a Martian (on Mars), you would still watch the Martian moons eclipse the Sun, but they are small and much closer to the Mars surface than our Moon is to Earth. From the surface of the red planet, the moons would not cover the entire disc of the Sun. Also, the Martian moons aren’t circular – so they would make potato shaped shadows across the surface!
Back on Earth, when a total solar eclipse occurs, it has profound biological effects. The animals are tricked into thinking it is night, and crickets will begin to chirp. If you ever get the chance to experience one, it would be well worth making the trek to see it. The next total eclipse visible from New Zealand is in July 2028 and will be best seen from the lower South Island. But be warned! You must always have your solar viewing glasses, as looking at the Sun for even a split second can cause significant damage to your eyesight.
We also witness partial, annular and hybrid eclipses. A partial eclipse is when the Moon takes a bite out of the Sun. You can still watch these (with your solar glasses on), but they aren’t nearly as awe-inspiring as a total solar eclipse. An annular eclipse occurs when the Moon is at apogee (the opposite of perigee), meaning it is the farthest away from Earth that it can get, and its disc appears smaller. When the Moon is at apogee and moves in front of the Sun, it obscures most of it, but there is still a circle of the Sun around the Moon.
A hybrid eclipse occurs when the position of the Moon between Earth and the Sun is so finely balanced that Earth’s curvature affects what type of eclipse occurs. From some places on Earth, the Moon will be just farther away from Earth’s surface, resulting in an annular eclipse. From other locations on Earth, the Moon will be just that little bit closer, meaning you’ll witness a total eclipse.
A sight to see
The other major eclipse is a lunar eclipse. This occurs when Earth moves between the Moon and the Sun and causes our little blue planet’s shadow to fall onto the Moon.
Lunar eclipse
Using different camera exposures, 1/80, 2/5 and 2 seconds, the two regions of a lunar eclipse can be seen. The outer penumbral shadow is where the sunlight is dimmed and the inner umbral shadow is where the much dimmer sunlight can be seen as a red colour as it reflects through the Earth’s atmosphere.
If Earth were a solid rock moving in front of the Sun, the Moon would become blacked out during a lunar eclipse. But this is not the case – Earth has an atmosphere! This means, when we see a lunar eclipse from Earth, the Moon appears to turn red. This is where the term blood Moon comes from.
But why does this happen? It’s all to do with light and the atmosphere. The sky is blue because, when the Sun’s light hits our atmosphere, it scatters more blue light across the sky than the other colours (light from the Sun is made up of all the colours found in a rainbow). Red light is seen often as the Sun goes down because we are seeing it through thicker parts of the Earth’s atmosphere. So when the Moon goes into Earth’s shadow, you see more red light, and this creates a blood-red colour – quite a sight to see!
Unlike solar eclipses, lunar eclipses are completely fine to view with the naked eye. They always happen in the middle of the night. In fact, there is a lunar eclipse happening next year. If you’re up in the early hours of the morning on 1 February 2018, you can witness one for yourself.
Useful links
See this great clip of a lunar eclipse occurring.
This video from National Geographic gives a great summary of all the types of solar eclipses.
The total solar eclipse that occurred earlier this year in the USA may be over, but check out the official eclipse web page from the event here, so you can get prepared for the 2028 New Zealand viewing!
This website has lots of information on eclipses including an animation, where to see them and interactive maps.
Believe it or not, the Babylonians were able to foretell eclipses 2,500 years ago! This Sky & Telescope article explains the saros cycle, and how ancient astronomers made use of it. Excellent history of science information.
Acknowledgement
This article has been written by Stardome Observatory and Planetarium, which has been operating since 1967. It is a place of exploration, research and sharing of knowledge and hosts New Zealand’s first and still largest planetarium theatre. Stardome Observatory and Planetarium celebrated its 50th anniversary in 2017.
