Understanding Solar Eclipses and How to Watch it
By Dr. N. T. Jiwaji
The major celestial event soon to occur over Tanzania this September 1 provides a very useful learning opportunity for all Tanzanians, young and old, casual observers and scientists. It is an opportunity to learn science and understand nature. By observing the eclipse safely, it will calm those who fear such events by participating in observation of the eclipse and understanding exactly what is happening. Let us explain what eclipses are and how they happen out there in space high above us. In fact this eclipse is awaited by people over the whole of Africa.
The word eclipse means to hide. So high above us, a solar eclipse occurs when the Moon hides all or part of the Sun from our view. But how can the Moon hide the Sun? There are two problems here. One is that the Moon is a very small body when compared to the Earth or even the Sun. Fifty Moon would fit inside Earth. But that is not all. The Sun is so huge that 50 million Moons could be swallowed by the Sun. How then can the Moon hide the Sun?
It is puzzling if we just think in terms of how big the Sun is, and how small the Moon is. But you should also understand that the Sun is extremely far away from us – 150 million kilometers away. The Moon is however, only 400,000 kilometers away from us. That means the Sun is 400 times further away from us than the Moon; that is if the Moon was 1 m away from us then the Sun would have to be placed half a kilometer away from us. So now you might begin to understand why the Moon can hide the Sun.
If you stretch your litter finger out at arm’s length towards the Sun, you can completely hide the Sun from your view. Your little finger has been able to cover the huge Sun!! In fact, only half the little finger is sufficient to cover the Sun. Sizes of distant objects are more easy to think of by how much angle they make at our eye. Hence for objects that are far away from us, we speak of angular size of objects.
The huge size of the Sun, which is also so very far away from us, makes the angular size of the Moon the same as the angular size of the Sun, when they are both viewed from Earth. When you look at the Sun through a solar viewer (never look at it any other way), you will think you are looking at the Moon because our mental picture of a full Moon in the night sky will be very very similar to the view of the Sun through the solar viewer.
In fact their angular sizes are almost exactly the same, that is, half a degree. This because though the diameter of the Sun is 400 times bigger than the diameter of the Moon, the Sun is also 400 times farther away from us than the Moon. This makes their angular sizes close to EXACTLY the same. This is in fact why solar eclipses occur and this is why solar eclipses are so interesting. If our Moon was much smaller it would not be able to cover the Sun at all. It is quite an amazing coincidence of nature that the Sun and the Moon are in exactly the same ratio of their distance to diameter and hence causes spectacular eclipses.
The other reason why the Moon can cover (that is, eclipse) the Sun is because it lies in an almost exact straight line with Earth and Sun. This can happen because the Moon circles the Earth in the same flat plane as the Earth revolves around the Sun.
Hence all three, Earth, Moon and Sun all lie on one huge flat empty surface. So during the Moon’s motion around the Earth, it can between the Sun and Earth hence eclipsing or hiding the Sun. One might ask if that is so, why does the Moon not hide the Sun every month because it circles every month and come between the Earth and Sun once each month. But this does not happen because the Moon, Earth and Sun are not always in the exact same flat plane. The Moon’s orbit goes up and down very slightly from this flat surface. Hence eclipses cannot occur every month. However, you should understand that there are on average two eclipses of the Sun every year (and also an average of two lunar eclipses each year), because our solar system is all in a quite a good flat plane and our mental picture of our Solar System should be a flat system, with planets and moons going up and down slightly above and below the plane as they go around the Sun.
Hence now we can appreciate why eclipses occur. Now let us look at the different types of eclipses. The most common type of eclipse that will be seen from large areas on Earth are Partial Solar Eclipses. This happens when the Moon covers only a part of the Sun so that a large part of the Sun can be left uncovered or a large part of the Sun can become covered by the Moon. This situation arises when the
Moon begins to cover the Sun and after its maximum the Moon moves continues to move gradually away from the Sun’s face. Millions of people across vast areas see some form of partial eclipse or the other during any solar eclipse. During this September 1 solar eclipse, people from the whole continent of Africa will be able to see the partial eclipse with those in the north and south of Africa seen only a small part of the Sun being covered, while those in central belly of Africa will see a large part of the Sun being covered. In Tanzania, millions of people will have the chance of seeing more than 90 percent of the Sun being covered.
In the central part of the eclipse region, we can see a Total Solar Eclipse or an Annular Solar Eclipse. Total eclipse occurs when the Moon is able to cover the Sun completely because the Moon’s angular size is at least as big as that of the Sun.
Total Solar Eclipses are only seen within a narrow 100-kilometer band in the centre of the eclipse region since that is the only part that the Sun’s shadow actually touches the surface of the Earth. The total part of the solar eclipse lasts only for a very short period just when their sizes match up exactly and it lasts only a few minutes, around three minutes. This is a spectacular occasion when during those three minutes, the daytime becomes night and animals and birds scatter just as we human being also get fear. However, it is a very short and we know why it happens and that the night will soon turn into day. During a total solar eclipse is the only time that we can get a chance to see the Sun’s atmosphere called the corona since the blinding light of the Sun is blocked from our view and we can study the corona from Earth. These three minutes of total darkness is the only time that you can see the Sun’s corona with the naked eyes. At all other times you cannot view the Sun with naked eyes because you will damage your eyes.
The distance from Earth to Moon changes slightly as it moves around Earth, since the Moon’s orbit is not an exact circle. Its distance varies from about 360,000 km (called perigee) to about 400,000 km (called apogee). Hence the Moon can sometimes be farther from Earth and sometimes it can be nearer to Earth. This means the size of the Moon, that is the angular size of the Moon, can change significantly. When the Moon is at apogee it will be slightly smaller than the angular size of the Sun, hence it is not able to cover the Sun completely when they are in eclipse position.
During this time we get an Annular Solar Eclipse, which is what we will see during maximum eclipse this September 1. Again this situation lasts only a few minutes, around three minutes because soon the Moon slides away from its central situation. Annular eclipses also occur only within a narrow 100-kilometer path on Earth where the Moon’s shadow reaches the ground on Earth. For this September 1, 2016 Annular Solar Eclipse, the 100-km path of annularity (where the ring of the Sun can be seen) passes from Gabon through Congo into Tanzania across Lake Tanganyika into Katavi, Rukwa, Mbeya, Njombe, Ruvuma and Mtwara Regions in Tanzania and then into Mozabique and crosses into the Indain Ocean onto Madagascar and then to Reunion islands.
During an Annular Solar Eclipse when viewed through a solar viewer you will see a bright ring (called the ring of fire) on the circumference of the Sun. Just when the Moon is about to enter or leave the edge of the Sun you can see points of light coming through the gaps in the mountains on the Moon. This is sight looking forward to during an annular eclipse.
From the beginning of the eclipse when the Moon begins to slide onto the face of the Sun and progresses across the face of the Sun until it leaves only a ring and then slides away from the Sun is the period when you will have seen the actual movement of the Moon in the sky. If you time the eclipse from when it just begins to touch the edge of the Sun until it finally slides its last edge away from the Sun, the Moon will have covered one degree of the sky, that is, one Moon-width and one Sun-width which makes total 1 degree. Hence you can calculate the angular speed of the Moon for it to move one degree. From this you can calculate how long it would take to move the full 360 degrees around Earth, which should work out to be 29.5 days. So this eclipse occasion is a time to understand the mathematics of our solar system also.
Hybrid eclipses occur when the distance between the Moon and Earth is close to its average value of 380,000 km. Then, the eclipse begins as an annular eclipse and then becomes a total eclipse for other parts of the Earth.
Your eyes will not allow you to look directly at the Sun because the light from the Sun is extremely bright so your eyes will immediately turn away and you should not force them to look at the Sun because you will easily damage your eyes and even blind yourself. The safest way to view an eclipse is using solar viewers which are specially made material which block 99.999 percent of the visible light and ultraviolet light.
However, not everyone can have access to these special solar viewers, but you should not use any type of coloured plastic or sunglasses or burnt sooted glass. Though your eyes might accept that much brightness, you can still damage your eyes without even knowing until a few days or weeks later. You can use welder’s glasses as filters as long as they are rated as number 13 or number 14 since these are dark enough to block all harmful light. Welder’s glasses rated 12 can be used briefly because the Sun will still look uncomfortably bright. You can ask for these at stores that sell welding materials but be careful not to be fooled by other ratings smaller than 12. Though they may look dark enough they can still pass harmful amounts of radiation.
An indirect easy way to view the shape of the eclipsed Sun is by making a small hole in a large cardboard or a large sheet of paper. The hole can be one or two millimeters wide. If you face the cardboard towards the Sun, there will be a large shadow on the other side and on the ground or on a screen you will see a small circle of light from the Sun. The shape of the normal Sun is round so you will see a sharp circle in the shadow of the cardboard. If you keep the cardboard more than 2 meters from the viewing screen, you will see a big enough circle.
During an eclipse you will see the shape of the Sun’s bright spot change from a circle to a crescent. If you are within the 100-km band passing through southern Tanzania, you will see the round circle change first to crescent then to a ring shape and again into crescent and finally a circle when the eclipse ends. This is a very easy way to make sure that you see how the eclipse progresses without any harm to your eyes. In fact any holes formed by natural objects will show the eclipsed crescent shape. Look for places on the ground where sunlight has passed through branches of trees (not coconut trees). Before the eclipse begins the bright spots will be round just as for the small hole, but during the eclipse you will see many crescents dancing on the ground. For fun take any object such a cloth or a cap which is loosely woven or has holes in it, and in its shadow, it will be fun to see the bright spots turning into crescents. You can even make holes in artistic patterns with enough spacing between holes and you will be able to make beautiful crescent patterns during the eclipse.
Timings for eclipse times for major towns and cities in Tanzania are as follows:
Many more details are about the September 1 Annular Eclipse in Tanzania and animations of how the eclipse will progress in the central band and different places in the country is available on: http://www.eclipseafrica.org/Other/TanzaniaList2016.html and you can get the timings for all districts also.
Go out and enjoy the eclipse and learn its science, and maths, and geography, and much more, so that you can become even more excited to discover the wonders of nature through science.
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