Via ABC News
We’re in for a real treat at the end of this month with a stunning red blood #moon right across Australia — the first total #lunar 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 #full 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.
