By Firitia Velt

There will be two lunar eclipses visible in Tonga this year. The first lunar eclipse coming up next week on the night 26-27 May is made for us, because the moon will be almost straight above your head. A full moon in a total eclipse is not pitch dark but usually bright red.

A lunar eclipse, ko e mate ʻa e māhina, happens when the full moon passes through the shadowcone of the earth.

Or said otherwise when, as seen from the moon, the earth passes in front of the sun, obscuring it so that darkness descends over the lunar surface. That is a total lunar eclipse to us.

For Tonga, the last one in February 2018 was a good one: the moon went very deep into the shadowcone, resulting in a dark moon and a duration of the total phase of more than a hour. And that all in the middle of the night with the moon high above the horizon.

The lunar eclipses since that time have been small and partial only. It is not until now, in 2021, that we finally get a good one again, in fact, two this year. Or do we?

The first eclipse will be in the night of 26-27 May, when the moon will be almost straight above your head. Or said otherwise, imagine a different perspective: any moon-dweller looking at us would see Tonga in the middle of the earth disk. Or maybe not, as they would be looking at the night side of our planet. Remember, the sun is behind the earth at that moment, so those moon-dwellers only see darkness.

And yet that is also not fully true, as they would still see a ring of red light around the earth: our twilight. Due to our atmosphere, sunrays bend around the earth. Bluish light is absorbed, only the reds are coming through. That is also the reason that we on earth see the reddish rising and setting sun.

But then, if some reddish light from earth hits the eyes of the lunarians, it is not really getting dark there. And we from earth see therefore that a full moon in a total eclipse is not pitch dark but usually bright red. But still dark enough to have all the stars visible in our sky, which during a normal full moon would be largely blotted out.

On 26 May the earth's penumbra starts touching the Moon's face at 21:47 pm. The partial eclipse begins at 22:44pm. The total eclipse phase lasts only 15 minutes: from 0:11am to 0:25am. Not much time for the moon to go deep into the shadowcone of the earth. In fact it will be very shallow, only 1% away from the rim at maximum. Expect to have a lot of twilight light leaking from the earth, most likely making this eclipse not very dark at all. Still worth looking, of course.

Second partial eclipse on 19-20 November

The next eclipse on 19-20 November 2021 will also be just-just, but then in the opposite direction. Now a tiny sliver of the moon will remain outside the shadowcone. Only 3% at best, but this will be enough to have this eclipse expelled from the league of totals. It will be a partial eclipse only. Still worth looking.

For 2022 we will see again two total lunar eclipses. The one in November will be a good one; the earlier one in May will be already almost over by the time the moon rises in the evening.

Not every full moon results in an eclipse, due to the tilted orbit of the moon around the sun. But usually twice per year it is straight over the sun, or at least almost so.

Sometimes it is skimming and the earth only passes over one side of the sun. Then not all the light is blocked and it is not really getting dark. From earth we see this as a partial eclipse. As about half on the lunar eclipses is partial only. A well visible, total eclipse of the moon only happens every couple of years on the average for any particular place on earth.

Is there any scientific value in observing eclipses? For solar eclipses, when the sun is obscured by the moon, the answer is decidedly, yes. Astronomical observatories have never hesitated to spend large amounts of money to send a telescope team to the most inaccessible places abroad to observe such eclipses. Tonga was among them. For example the total solar eclipse of 22 October 1930 which was visible in Niuafoʻou [2]. Or the one from 29 April 1911 in Vavaʻu [3].

Only during such an eclipse when the moon blots out the bright solar disk were astronomers able to observe its tenuous atmosphere. Nowadays however we have satellites, which being outside our atmosphere, can observe these outer layers continuously, reducing the need for chasing solar eclipses.

Historically lunar eclipses can help us to pinpoints events in the past. For example if historians disagree whether a particular battle took place on that or that date in ancient times. If then the chronicles tell, for example, that there was lunar eclipse visible the night before, that settles the case. Solar eclipses are even better in that respect, but they are much more rare.

Total Lunar Eclipse, in Nukualofa, Wed, 26 May 2021, 21:47 [4]

More information about the eclipse: [4]

Time and Date.com: [4]Map of Total Lunar Eclipse on 26 May 2021 [5]

Time and Date.com: How to Take Pictures of a Lunar Eclipse [6]

More information about the Moon:

Royal Museums Greenwich: These are the Moon's Phases [7]

Night Sky Pix: Why does the Moon Have Craters? [8]