Each month, at the time of new moon, the sun and moon are together in the daytime sky. Most of the time the moon passes by unnoticed.
But at least twice a year, somewhere on Earth will see the moon pass in front of the sun and the spectacular phenomenon of a solar eclipse occurs. (You can check when your part of the world will next see one on NASA’s solar eclipse catalogue.)
While sometimes the moon perfectly hits its mark and the sun is dramatically hidden from view in a total solar eclipse, it’s also possible for the moon to line up directly with the sun but not block it out completely.
In this circumstance, called an annular eclipse, a ring of sunlight shines out from around the dark moon.
But most commonly we experience a partial solar eclipse. In this instance, the moon’s path only partly crosses the sun and by using the right sort of equipment, it’s possible to see a chunk of the sun disappear behind the moon.
It comes down to coincidence that it’s at all possible for a total solar eclipse to occur. The moon is much smaller than the sun, but it’s also much closer. By diameter the moon is about 400 times smaller and by distance it’s roughly 400 times closer.
This perfect match means that to us on Earth, the sun and moon appear a similar size.
But there’s more. Anyone who’s been aware of the supermoon craze that has developed in recent years, will also know that the moon’s apparent size varies slightly throughout the month. This is because the moon travels along an elliptical orbit around the Earth. (The size of the sun varies too due to the Earth’s elliptical orbit, but it has a much smaller effect.)
The changing size of the moon affects what sort of solar eclipse will be seen whenever the moon passes directly in front of the sun.
If a solar eclipse occurs when the moon is near apogee (or furthest from Earth), then the moon will be appear too small to completely cover the sun. A thin ring of sunlight remains in view, creating an annular eclipse.
Since the moon is further away, it is the moon’s antumbral shadow that touches down on Earth. This section of the moon’s shadow extends beyond the umbra or the shadow’s darkest part.
A total solar eclipse occurs when the moon is close enough for the umbra to reach Earth. And the closer the moon is, the longer totality lasts. On rare occasions totality can extend for just over seven minutes. But most of the time it lasts only a couple of minutes or so.
Right time, right place
Whenever the sun, moon and Earth fall into line to produce a solar eclipse, there’s only a limited part of the Earth that is cast into shadow.
In 2012, NASA astronaut Don Petit captured a solar eclipse from on-board the International Space Station. From this incredible view point, it was possible to see the moon’s small shadow zip across the planet at nearly 2,000 km/h.
Petit’s images captured an annular eclipse and only places near the very centre of the shadow saw the sun become a ring of light. But the same is true for total solar eclipses also. It is only near the centre of the moon’s shadow that totality is seen.
All other regions within shadow see a partial eclipse. These places lie within the moon’s penumbral shadow that is much larger than the umbra or antumbra.
Sometimes, the alignment of the sun, moon and Earth is not perfect and nowhere on Earth sees the moon directly pass in front of the sun. This is a truly partial eclipse and only the moon’s penumbral shadow falls upon the Earth.
Interestingly, on rare occasions an eclipse can be both a total eclipse and an annular eclipse. It all depends upon where on Earth you are watching it.
Known as a hybrid eclipse, what happens is that the Earth lies very close to the sweet spot, between the umbral and antumbral shadows. Amazingly, the curvature of the Earth is enough to bring some places into the umbra to create a total solar eclipse, while others are a touch further away and fall into the antumbra to see an annular eclipse.
During a solar eclipse, the shadow cast by the moon is so small and moving so fast that the eclipse tracks a thin path across the Earth. Each place along the path sees the eclipse in its own time.
This is very different to a lunar eclipse, where the moon is engulfed in the Earth’s much larger shadow. Lunar eclipses are seen from the entire night side of the Earth and everyone, no matter where they are, witnesses the event at precisely the same moment.
Watch an eclipse, but don’t hurt your eyes
The most important thing to know about solar eclipses is that you cannot watch one without first protecting your eyes. The sun emits intense UV radiation which makes it harmful to look at the sun at any time.
An eclipse makes it more tempting to look up at the sun. But even though the moon may be blocking regions of the sun, enough UV radiation remains to cause permanent damage to your eyesight.
Don’t miss the chance to watch a solar eclipse but take the right precautions to do so.
Sometimes it’s a matter of looking around and letting nature lend a hand. Shadows made by a leafy tree on the ground or a wall (five to 10 metres away), can produce overlapping circular spots of light. These bright spots are images of the sun and during an eclipse they turn into crescents.
But to see the eclipse directly, one of the easiest methods is to purchase inexpensive but specially designed eclipse glasses from reputable telescope and astronomy shops or your local public observatory, science centre or planetarium.
Such organisations, as well as amateur astronomical societies, may also organise viewing events using special solar telescopes or by fitting solar filters to their usual astronomical telescopes.
The best DIY method is to create a pinhole camera. The simplest one involves making a small circular hole (with a nail or tack) in a large piece of cardboard. Do not look through the hole, but allow the sun to shine through and project an image onto a second piece of cardboard, or even a blank wall or clear patch of ground.
The spectacle of totality
It’s always intriguing to watch the moon take a bite out of the sun during a partial eclipse. But to experience a total solar eclipse is absolutely stunning. Furthermore, it is safe to watch an eclipse during the brief moments of totality when the sun is completely covered by the moon.
During the partial phase of an eclipse there is very little to make you aware that the eclipse is happening. The change to daylight is quite minimal until about 10 minutes or so prior to totality.
As totality approaches the surrounding light becomes somewhat sharp and unnatural. The temperature drops and animals can become confused.
If you watch for it, you can even catch the moon’s shadow racing along the ground towards you. Some surfaces can be covered with “shadow bands”, rippling waves of light and dark shadows.
The sun is now such a thin crescent that its light is being noticeably refracted or disturbed by the Earth’s turbulent atmosphere (such atmospheric disturbance is what makes stars twinkle).
Just before the moon fully lines up with the sun, the sun’s thin crescent is broken up into little dots of light. They are called Bailey’s beads, formed by sunlight shining through the uneven surface of the moon.
Moments later there’s a final bright flash as the last of the sun’s light flares out through a lunar valley. This brilliant spectacle is known as the diamond ring effect.
With the bright sun hidden behind the moon, eye protection is no longer needed and it’s possible to see layers of the sun that are normally hidden.
The sun’s outermost atmosphere, called the corona, extends millions of kilometres out into space. The tenuous gas is delicate and wispy; it’s a stunning sight.
There is also colour around the edge of the moon. These pink prominences are part of the sun’s chromosphere or “sphere of colour” and they dance around, changing with every moment.
If you can tear your eyes away from the sun, then the sky is dark enough to see the brightest stars. Often Mercury or Venus are also visible, as these two planets never drift very far away from the sun.
Totality ends with another diamond ring effect and all too quickly eyes are shielded again and it’s time to watch the strange twilight come to an end. It’s an experience well worth seeing. And for some people, known as eclipse chasers, they will travel the world to catch every moment of totality wherever it may be.
A solar eclipse doesn’t happen at every new moon because the moon’s orbit is tilted by 5 degrees relative to the Earth’s orbit around the sun. Most of the time, the moon’s shadow misses the Earth, passing either above or below it.
But around twice a year, there is an eclipse season. Lasting about a month, the orbits are aligned so that eclipses can occur.
During this time we see a pair of eclipses, consisting of a solar eclipse (at new moon) and a lunar eclipse (at full moon). And since the new moon and full moon phases vary by about a fortnight, it’s even possible for there to be a trio of eclipses in one season.
Most of the time, each eclipse within a season is visible from different parts of the world.