Eclipses. Solar and Lunar

Transcription

1 Eclipses Solar and Lunar

2 An eclipse occurs when one body comes between the sun and a nearby body such that the shadow of one falls on the other. A total eclipse is when one body is seen completely occluded from the other. A partial eclipse is when part of the occluded body remains visible. Notice that the sun never passes between two bodies. For one thing, the sun is far too big to pass between any objects that could experience an eclipse with each other without turning them both to cinders. For another, the sun doesn t move at all, at least within the solar system. We are interested in eclipses involving our moon and Earth, but other planets have moons and can experience eclipses, though nobody is around to see them.

3 This model, which you should realize is not to scale, reminds you that, rotation around a common barycenter notwithstanding, the moon effectively orbits Earth. Thus any eclipse involving the two bodies will necessarily be because the moon has moved into the proper position, not the Earth. The view is from above the North Pole of Earth. There are two, and only two, places in the lunar orbit where a shadow of one object can be cast on the other. At point A the shadow of Earth can be cast on the moon and at point B the shadow of the moon can be cast on Earth. Make sure that you see that these are the only two places for the moon that might result in an eclipse. A B

4 With the moon in position A the shadow of Earth will darken the face of the moon and a lunar eclipse results. With the moon in position B the shadow of the moon will darken Earth and block the sun from our view, making it dark during the daytime. This is what happened at GSW in the Fall, 2017 semester. That was a partial eclipse here, meaning we always saw at least a bit of the sun, and it never got completely dark. A little farther north is was a total eclipse. The sun was entirely blocked from view and complete darkness resulted. A B

5 The moon gets into these two positions every (sidereal) lunar month. Remember that in position A the moon is full, and in position B it is new. These are the only two phases of the moon in which an eclipse can occur. Just in case you need reminding, position Z explains why. So why is there not an eclipse every time there is a full or a new moon? To understand this you have to remember that this is a three dimensional system. Viewed from the north we only see two of them. Moon s shadow goes that-a-way Z A B

6 The ecliptic is the plane in which the Earth/moon system revolves around the sun, and therefore marks the direction to the sun from us. Remember that our axis is tilted by about 23.5 from a perpendicular to the ecliptic. Now assume that the moon orbits directly above the equator. In the configuration shown notice that at full moon ( A )The shadow of Earth would be south of the moon, and at new moon ( B ) the shadow of the moon would be south of Earth. (The lack of scale is not our friend here. The shadows would literally miss Earth and the moon in this configuration because of the scale.) A 23.5 ECLIPTIC B

7 In point of fact the moon does not orbit directly above the equator, but at a roughly 28.5 angle from it as shown. If you watch the moon night by night for a month it will appear to move far to the north (coming to lie almost overhead at Americus) and then far to the south 28.5 north of the equator and then 28.5 south, and back, and forth. The moon does not wobble. This appearance of wobbling from Earth is because we turn a little faster than the system does, so we see the moon at different heights day by day because we turn under the 28.5 slant little by little, just as we turn under the tidal bulges. Even with the scale problem it should be very obvious to you that Earth cannot cast a shadow on the moon at A (new moon) and the moon cannot cast a shadow on Earth at B (full moon). They are in the wrong places. However, at Y there could be a solar eclipse in the northern hemisphere and at Z there could be a lunar eclipse in the southern hemisphere. A 23.5 Z 57 Y ECLIPTIC B

8 What is shown in the previous diagrams has been the configuration of the system at the summer solstice, with the axial tilt in the northern hemisphere toward the sun. With a little thought you should be able to see that an identical situation (except that the lunar and solar eclipses would be reversed) would hold at the winter solstice. Think of the Earth/moon system behind the sun instead of in front of it. The situation would be identical. What about at an equinox either one. In the configuration shown (full moon) notice that the moon will be on the equator, and in full shadow, no matter which part of its orbit it is in far north or far south of the equator. With a little thought you should be able to see that at new moon the same would be true of lunar eclipses A ECLIPTIC

9 At the solstices the situation for eclipses of either type is maximally bad. The moon/earth alignment is most likely to place the moon too high or too low for an eclipse. At the equinoxes the situation is optimally good. The moon will pass directly between Earth and sun. In between, as you should be able to imagine, the situation is also in between maximally bad and maximally good. The closer to the equinox you get the better the chances of an eclipse; the closer to the solstices, the worse they get. But even at the equinoxes ther is no guarantee that an eclipse will happen because there is no guarantee that a new or full moon will coincide with the equinox! No matter how favorable the angles are for an eclipse, if it s either a waxing or a waning moon there will be no eclipse because eclipses can only happen at new and full moons.

10 PARTIAL SOLAR ECLIPSES: 1) In the configuration shown there will be a shadow of the moon cast on Earth near the south pole, as shown. Everywhere north of the indicated solar ray will not see an eclipse at all. But even within the shadow observers will always be able to see part of the sun. At S, for example, an observer would not be able to see the bottom of the sun, as the broken line-of-sight arrow shows, but they would be able to see the top of it, as the unbroken arrow shows. This will be a partial eclipse for all observers. ECLIPTIC Lunar Shadow S Solar ray that grazes the moon

11 PARTIAL SOLAR ECLIPSES: 2) Because the moon is small and far away its shadow does not fully cover Earth at an eclipse. In the configuration shown the lunar shadow will cover a fair bit of Earth, but not all by any means. Observers within that shadow zone will se an eclipse of some sort, but only those between the two red arrows will see a total eclipse. Anyone within the shadow north or south of this will see only a partial eclipse. People north of the northern arrow will see the north part of the sun and those south of the southern arrow will see the southern part. This diagram too has a scale problem. It is not obvious from this diagram, but anyone north or south of the shadow will not see an eclipse at all, not even a partial. The problem is that we cannot accurately show the distances nor the relative sizes of the objects accurately. ECLIPTIC Lunar Shadow Solar ray that grazes the moon Solar ray that grazes the moon

12 PARTIAL LUNAR ECLIPSES: Partial lunar eclipses only happen when the moon and Earth are not quite aligned, as in this configuration. All observers on Earth will see the roughly same thing in this case the southern part of the moon in shadow. Some will see a little more shadow and some a little less, bur everybody will see a partial eclipse. If they re on the correct side of the Earth and it s nighttime. What would it look like if the moon were below the Earth with respect to the sun? Because the Earth is so much bigger than the moon, its shadow, even at the distance of the moon, is over twice the moon s diameter. This means that whenever the moon is entirely behind Earth between the indicated solar rays there will be a total lunar eclipse. Every observer on Earth will see a full total eclipse, no partial is possible because the entire face of the moon will be in shadow. Earth s Shadow on moon Solar ray that grazes Earth ECLIPTIC Solar ray that grazes Earth