Thousands of people are flocking to Exmouth in Western Australia this week hoping to catch a solar eclipse.
This tiny, remote town and surrounding peninsula will be the only place in Australia to experience a total solar eclipse on April 20.
It is part of a rare hybrid eclipse (more on that later) that will occur along a narrow track that extends from near Antarctica to the Marshall Islands in the Pacific Ocean.
Exmouth lies directly under the shadow that will be cast by the Moon as it lines up perfectly with the Sun.
During this time, known as totality, the Sun will be completely blocked out for 58 seconds before it reappears.
Meanwhile, the rest of Australia will experience a partial solar eclipse, where the Sun is never completely blocked out.
How much of the Sun is covered, and whether or not you will notice it at all, will depend upon where you live.
"This eclipse is really one that favours Western Australia and the Northern Territory," said astronomer Jonti Horner of the University of Southern Queensland.
In the first of a series of stories explaining the eclipse, let's run through the different types of eclipses and what to expect across Australia.
But first a warning: it is not safe to directly look at the Sun, particularly during a partial solar eclipse or the partial phase of a total eclipse, without using eye protection that meets the Australian standard ISO 12312-2 and is undamaged.
"Your eyesight can be damaged before you notice it," Professor Horner said.
We'll look at ways to safely view the eclipse using indirect methods and take photographs in another story.
What is a solar eclipse?
Solar eclipses occur when the Moon passes between Earth and the Sun, blocking out all or some of the light from the Sun.
Between two and five solar eclipses happen every year, casting shadows on different parts of the Earth each time.
These spectacular events occur during the new moon phase, but they don't happen every month because the Moon's orbit is not a perfect circle and is tilted so it doesn't always line up.
"Any point on the Earth can get an eclipse, but it comes down to the nuance of the timing of exactly when the Moon and the Sun line up," Professor Horner said.
There are three main types of eclipse:
- total eclipse
- partial eclipse
- annular eclipse.
A total eclipse occurs when the Sun, Moon and Earth are perfectly aligned and the Moon appears to totally block out the Sun.
As the Moon moves in front of the Sun, it casts a shadow on Earth.
The central part of this shadow is called the umbra, and the outer part of the shadow is called the penumbra.
The shadow moves along a path created by a combination of the changing position of the Moon in the sky and the Earth's rotation.
Places on Earth that fall within the umbra see a total eclipse for a short period, while places that fall within the penumbral shadow see a partial eclipse.
Partial eclipses are part of a total eclipse — appearing before and after totality in places that lie on the central path — or for the entire duration in areas that aren't under the umbra.
They can also happen independently of total solar eclipses when the orbits of the Sun, Moon and Earth don't match up perfectly.
An annular or "ring of fire" eclipse is similar to a total solar eclipse, except the Moon doesn't fully cover the Sun so a ring of gold appears around the darkened silhouette.
This happens when the Moon is slightly further away from us, so the umbra doesn't reach Earth.
Instead, the front part of the shadow, known as the antumbra, moves along the eclipse path.
So what's a hybrid eclipse?
A hybrid eclipse is a combination of a total and an annular eclipse.
Hybrid eclipses make up only about 3 per cent of all solar eclipses.
The last hybrid eclipse anywhere in the world was on November 3, 2013. The next is not until November 14, 2031.
Hybrid eclipses occur when the Moon is at a precise distance away that its shadow is affected by the curvature of the Earth.
"The rarity of this eclipse occurs because the distance has to be exactly right," Professor Horner said.
The eclipse shifts from being annular, to total, and back to annular at the end.
"When it's close enough you get a total lunar eclipse, when it's further away [because of the curvature] you get an annular eclipse.
"If it was a little bit closer, everywhere on the path would have a total eclipse. If it was a little bit farther away, everyone would have an annular eclipse."
This month's eclipse starts out near the Kerguelen Islands, above Antarctica, as an annular eclipse that lasts just seconds.
As the shadow moves north it becomes a total eclipse, grazing the west coast of Australia before travelling up to East Timor, where it peaks with a maximum totality of 1 minute and 16 seconds.
Then it continues up to Papua, before moving into the Pacific where it becomes an annular eclipse again along a very narrow slice of ocean near Kiribati.
What will happen in Exmouth?
Exmouth just happens to be bang under the shadow path.
The eclipse, including the partial phases, will last for 2 hours, 58 minutes and 3 seconds in total.
Totality will last for about 58 seconds.
"This is quite a short eclipse," Professor Horner said.
During this time, the face of the Sun will be completely covered by the Moon, producing some spectacular effects (which we talk about in the next story).
Before and after totality, the Sun will gradually disappear and reappear as the Moon moves across it during the partial phase of the eclipse.
Partial begins (AWST) |
Totality begins |
Max |
Totality ends |
Partial ends |
|---|---|---|---|---|
10:04:31 AM |
11:29:48 AM |
11:30:17 AM |
11:30:46 AM |
1:02:34 PM |
What about everywhere else?
The path of the central shadow is only about 40 kilometres wide as it passes over Exmouth.
"If you are right on the edge of the shadow you get less totality because the alignment is not as perfect," Professor Horner said.
That means everywhere else in Australia will experience a partial eclipse, with the Moon covering the Sun to varying degrees ranging from between 80 – 95 per cent in the north and west, down to less than 5 per cent in Tasmania.
"In Darwin you'll get an 80 per cent eclipse, so by mid eclipse you'll have a crescent sun in the sky and it will be spooky," Professor Horner said.
"Similarly, in Perth it'll be about 70 per cent, so there'll be a distinct crescent."
But the eclipse will become less and less noticeable anywhere further east of the north-western corner of South Australia, where around half the Sun is covered.
Location |
Maximum partial eclipse |
% covered |
|---|---|---|
Broome |
11:52:32 am (AWST) |
89 |
Darwin |
1:52:28 pm (ACST) |
81 |
Perth |
11:20:30 (AWST) |
71 |
Alice Springs |
1:37:48 pm (ACST) |
48 |
Cairns |
2:29:21 pm (AEST) |
42 |
Adelaide |
1:30:02 pm (ACST) |
21 |
Brisbane |
2:44:56 pm (AEST) |
16 |
Melbourne |
2:09: 05 pm (AEST) |
11 |
Sydney |
2:28:56 pm (AEST) |
10 |
Canberra |
2:22:10 pm (AEST) |
10 |
Hobart |
2:06:56 pm (AEST) |
5 |
"Realistically, in New South Wales, Victoria, the Australian Capital Territory, Tasmania and the south-eastern part of Queensland, you're not going to notice much."
This is not just because of the percentage of the Sun covered, but also the way our eyes perceive changes in brightness of the surrounding landscape.
"Our eyes are an incredible tool because they can see light and dark at the same time, but that means they're not very sensitive to changes in brightness," Professor Horner said.
"So when the Sun is half eclipsed ... we will be getting 50 per cent less light, but you wouldn't tell.
"The scenery around you looks just the same, and you won't really notice a difference."
How does this eclipse compare with others?
The longest period of totality during an eclipse ever recorded lasted 7 minutes and 32 seconds.
At 1 minute and 16 seconds at its peak, this eclipse is just a brief taster of what is to come.
It is the first — and shortest — of five eclipses that will happen over Australia in the next 15 years.
The next eclipse, on July 22, 2028, will be a total eclipse that cuts a path down from the Kimberley across to Sydney.
At the peak of totality in the Kimberley, the Sun will be covered for 5 minutes and 10 seconds. Everywhere in Australia outside the path of totality will get a partial eclipse of at least 50 per cent.
There will also be:
- a total eclipse across south-eastern Australia on November 25, 2030
- an annular eclipse that goes through Tasmania on March 9, 2035
- a total eclipse running across the centre of Australia on July 13, 2037.
Solar eclipses vs lunar eclipses
Solar eclipses always go hand-in-hand with lunar eclipses.
Lunar eclipses fall two weeks before or after a solar eclipse and vice versa.
While solar eclipses fall during the new moon phase, lunar eclipses fall during the full moon phase.
This is because there are two points in the sky 180 degrees apart where the Moon's orbit around Earth lines up with the Earth's orbit around the Sun.
"When the Sun is near one of those crossing points, there's the chance that, when the Moon reaches that crossing point, the two will line up," Professor Horner said.
During a solar eclipse, the Moon passes in front of the Sun, casting a narrow shadow that can only be seen from a small part of Earth.
"The Moon is smaller than Earth, so it casts a smaller shadow," Professor Horner said.
But during a lunar eclipse, the Sun is behind the Earth, which casts a shadow across the Moon that can be seen from anywhere on Earth where it is night time.
Again, the type of lunar eclipse you see depends on how well the Sun, Earth and Moon line up.
On May 5, two weeks after this solar eclipse, there will be a penumbral eclipse of the Moon.
At best you might see the Moon get a tad greyer as it passes through the Earth's outer shadow, but unlike the solar eclipse, it is safe to look at.
