Via ABC News

We’re in for a real treat at the end of this month with a stunning red blood right across Australia — the first total eclipse since 2015. But what is a red blood moon?

With our beginner’s guide, red blood moons, blue moons, supermoons, lunar phases, and — of course — the ‘man in the moon’ will all become as plain as day.

Lunar phases

The Moon orbits the Earth every 27.3 days — which is the same amount of time it takes to spin on its axis, and this is why the same side of the Moon always faces us.

But two things change about the Moon’s appearance every day — how much of it is lit up by the Sun (giving us the phases), and what time it rises in the east.

It’s a common misconception that the phases are caused by the Earth’s shadow falling on the Moon (that’s actually what happens in an eclipse — see below).

But the dark parts of the Moon are dark for the same reason the night side of Earth is dark — they’re not being illuminated by the Sun.

As the Moon orbits Earth, it changes its angle to the Sun, relative to the Earth. A complete lunar cycle — the time taken for the Moon to go through its various phases and return to the same position — takes around 29.5 days.

During a moon the Moon is on the exact opposite side of the Earth to the Sun, so we see it fully lit. And being opposite the Sun, it rises at the very viewer-friendly time of sunset.

Because the Moon’s orbit takes slightly less than a calendar month, we get to see two full moons in a month about once every 2.7 years. This is called a ‘blue moon’.

A few times in a century, two blue moons can occur in a year. This year there will be blue moons in January and March for most Australian time zones — the first time this has happened since 2010.

The day after a full moon, the Moon is slightly less full (waning) and rises a bit later. And every day the lit area gets thinner, reaching the last quarter moon a week later. By this stage the Moon rises pretty late, so only night owls are likely to see it.

By the time of the new moon — a fortnight after the full moon, and the true start of the cycle — moonrise is so late it actually rises and sets with the Sun, so we never see the new Moon at all. Except during a solar eclipse, when it moves between us and the Sun.

The combined gravitational pull of the Moon and Sun when they are aligned at full and new moon produces spring tides — our highest tides. The lowest tides, called neap tides, occur when the Moon and Sun are at 90 degrees to one another, with respect to the Earth.

Lunar eclipses

Lunar eclipses occur when the Earth’s shadow falls across the Moon.

If the Moon’s orbit was perfectly aligned with that of the Earth and Sun we’d have a solar and lunar eclipse with every full and new moon — when the Sun, Earth and Moon line up.

But sadly, the Moon’s orbit is tilted by about 5 degrees. That means lunar eclipses only happen during full moons when the Earth’s and Moon’s orbits intersect.

This happens twice a year, but slight wobbles in the Moon’s orbit mean we will not necessarily get to see the same type of eclipse each time.

There are three kinds of lunar eclipses: total, partial and penumbral.

Total eclipses happen when the darkest part of the Earth’s shadow — the umbra — passes directly over the Moon, turning all of it a shade of red, aka ‘blood moon’. The reddish colour is caused by the filtering and bending of the Sun’s light through our atmosphere. The Earth’s atmosphere absorbs blue and green light so only red is left in our shadow.

Partial eclipses happen when part of the Earth’s umbral shadow passes over the Moon, blocking some of the light from the Sun. This turns part of the Moon dark.

Penumbral eclipses happen when the outside part of the Earth’s shadow — the penumbra — passes across the Moon. This makes the Moon look slightly darker but the difference is hard to detect.

When an eclipse takes place, everyone on the night side of Earth can see it at their equivalent local time.

You may see a penumbral and partial eclipse on either side of a total eclipse — just what phases you see will depend upon your time zone.

On average a total lunar eclipse can be seen every 2.5 years, but sometimes you can get a run of them. There will be three total lunar eclipses between January 2018 and February 2019 followed by a partial eclipse in July 2019.

Other times you’ll get a run of partial-only or penumbral-only eclipses.

The NASA website has a list of lunar eclipses that goes up to 2100.

You can see any lunar eclipse as long as the sky is dark enough, the Moon is high enough in the sky to see above the horizon, and the weather is clear.

The ‘man in the moon’

There’s no official “man” in the Moon — but it’s possible to spot a man’s face in any full — or even full-ish — moon.

One thing’s for sure, it won’t be the same man they see in the northern hemisphere.

In Australia, the Moon is “upside down” from the point of view of northern hemisphere viewers. We see a jolly man’s face in the full moon, while their guy looks a bit alarmed.

The dark and light patches on the Moon’s surface are reminders of its hectic past.

The Moon formed from material blasted out of the early Earth, when it was hit by something the size of Mars 4.5 billion years ago. The young Moon was pummelled by asteroids which gave rise to the dark patches (cooled lava) and lighter patches (craters) we see today.

The Moon would have looked the same to the first humans, and even the dinosaurs if they glanced upwards.

While we don’t know if our ancestors saw a face, they thought the dark patches on the Moon were seas, and they named them accordingly, like the Sea of Tranquillity where humans first landed on the Moon (our southern moon man’s right eye).

Look up! It’s a supermoon

As well as being slightly tilted, the Moon’s orbit is an ellipse (egg-shaped), and that means the Moon’s distance from Earth varies from closest approach (perigee) to furthest away (apogee).

At this closer distance (356,357 km), a supermoon looks about 15 per cent bigger and up to 30 per cent brighter than a full moon at apogee (406,720 km).

Some people call the closest moons “supermoons”. This is not an astronomical term but rather a name coined by an astrologist in 1979.

A full moon at perigee happens around every 14 moons (13 months), so that’s how often we should see a supermoon.

In reality, the difference in the size and brightness of the Moon at its closest and furthest points is too small to be seen with normal human vision — you need telescopic images for that.

Recently, however, full moons near perigee — such as the January 31, 2018 full moon — have been called “supermoons” too, which is why we can have three full moons in a row dubbed ‘super’.

Lunar tricks

The full moon also looks much bigger when it is on the horizon than when it is higher in the sky. But don’t be fooled — and don’t get it confused with a supermoon. This is an optical illusion.

We’re not really sure why our brains make the horizon moon look up to twice as big as the high moon.

One hypothesis is that we’re rubbish at estimating vertical distances — we think the Moon is smaller and closer to us at its highest because we perceive sky as a flattened dome rather than a sphere.

Another idea is the way we use reference points such as objects in the foreground and background to judge distances — but we have no reference points for things above our head so the high moon looks smaller than the moon on the horizon.

If you need proof it’s an illusion, NASA suggests taking a close up photo with a telephoto lens when the Moon is on the horizon and a couple of hours later when it is high in the sky. The horizon moon will be a little bit vertically squashed but the horizontal diameters of the two moons will be the same.

The horizon moon will also look a little warmer than the high moon, thanks to the light-bending effect of Earth’s atmosphere.

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