SEPTEMBER 16 PENUMBRAL LUNAR ECLIPSE

SEPTEMBER 16 PENUMBRAL LUNAR ECLIPSE

Millions of Tanzanians witnessed the exciting solar eclipse two weeks ago on September 1. Now, two weeks after that, another eclipse will occur over Tanzania on the night of September 16, 2016 between 8 pm to 1 am past midnight. This is a lunar eclipse, while on September 1 we had witnessed a solar eclipse. Lunar eclipses can be seen by half the world so many more people can see a lunar eclipse than solar. You do not need any glasses to see a lunar eclipse since we all see the Moon using our naked eyes and during an eclipse the Moon becomes even dimmer.

A penumbral eclipse is not easy to notice because it is only a faint penumbral shadow. However, this lunar eclipse is worth going out to look since it comes just two weeks after our well watched annular solar eclipse of September 1. This lunar eclipse will begin at just before 8 pm when the Moon begins to enter the light penumbral shadow of Earth and maximum eclipse will occur at just before 10 pm when the Moon almost completely submerged in the Earth’s penumbral shadow. After that it begins to moves out and the eclipse ends after midnight just before 1 am.

During the last annular solar eclipse on September 1, we were able to see the movement of the Moon across the face of the Sun. Since then the Moon continues to move further in its orbit around the Earth it will have made half the orbit by 16 February which will bring it in line again with Sun and Earth but on the other side, so that the Earth will now be between the Sun and Moon.

By September 16, the Moon will be a bright disk in its Full Moon phase. So you can see that solar eclipse always occurs during a New Moon while a lunar eclipse always occurs during a Full Moon.

We understood that during a solar eclipse the Moon comes in front of the Sun and the Moon’s shadow falls on Earth. The light from the Sun is blocked by the Moon and we see the Sun as a crescent, or as a ring during the annular eclipse, or completely covered during a total solar eclipse. And for this to occur, the Moon had to be in an exact straight line between the Earth and the Sun. Now, two weeks after our annular solar eclipse, the Moon still remains almost exactly in that straight line even after it has moved by half its orbit but now the Earth will be between the Sun and the Moon, resulting in a lunar eclipse. This is why a solar eclipse are often followed by a lunar eclipse, or vice versa, but in a different parts of the world.

Our solar system is a very flat system in which all the planets revolve around the Sun in this flat plane. The orbit of the Moon around the Earth is also almost in this same flat plane but not exactly. If the Moon was exactly in the same plane as the planets, then it would block our view Sun during each orbit and we would have one solar and one lunar eclipse every month! Of course this does not happen because the orbit of Moon is slightly inclined, so on average there are only two solar and two lunar eclipses per year somewhere on Earth.

Eclipses are about shadows falling on Earth or the Moon. The different types of eclipses depend on the types of shadows. One may think that a shadow is just a shadow, so what do we mean by types of shadows? Look closely at the shape made by your shadow as you walk in sunshine and on the edges you will see a faint border around your shadow.

The faint shadow is called a penumbra while the main shadow is dark and is called umbra. Umbra and penumbra are formed when the source of light is broad. A point source would produce umbra only. A large source would have a large penumbra while the umbra would be small. A penumbra shadow is formed because light from one part of the source is able to enter into the shadow formed from the other opposite part of the source.

The Moon’s shadow from the Sun is a small thin cone that extends up to the Earth and produces a solar eclipse. However, since the Earth is large, its shadow forms a very large cone that extends well beyond the Moon. When the Moon enters this shadow a lunar eclipse occurs and since the shadow is large, everyone on the night side of Earth can see a lunar eclipse. That is why many more people will have seen a lunar eclipse than a solar eclipse.

he Moon can fit in this shadow in many different ways depending on whether it is in the penumbra or the umbra and this produces different types of lunar eclipses. When the Moon is completely inside the Earth’s umbra, a total lunar eclipse occurs and the whole face of the bright Moon is covered in Earth’s shadow. When the Moon is only partly inside the umbra a partial lunar eclipse occurs. Of more interest to us this time is the situation where the Moon remains only in the penumbra. Then we get a penumbral lunar eclipse which is what will occur on in a few days on the night of Friday, September 16.

This lunar eclipse is a deep penumbral eclipse since more than 90 percent of the Moon will be covered by the Earth’s penumbra. The eclipse will begin at 7:55 pm when the edge of the Moon first enters the penumbral shadow of Earth. It is quite difficult to see this phase because the penumbral shadow is quite light. Get used to how bright the Moon is without any shadow. Then, during the next two hours the Moon will become gradually less bright as it slides eastwards into the penumbral area and by 9:54 pm almost the entire Moon will be covered by the penumbra. This is maximum eclipse and you will be able to see the Moon has clearly dimmed when compared with the brightness of the Full Moon that you saw at the beginning of the eclipse.

The penumbral shadow of Earth is slightly yellowish since some light is scattered from the Earth’s atmosphere. The side of the Moon that is facing north will become distinctly dark since it is near the dark umbral shadow while the side of the Moon that if facing south will be very bright since it does not enter the penumbra at all. It is quite a satisfying challenge to notice these differences in the brightness of the Moon and you will have been able to see effect of Earth’s penumbra on the Moon.

Figures with Kiswahili captions: