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Eclipse Q&A - Page 1

This section contains a lot of common questions regarding eclipses and the related answers. While you are reading the following, our eclipse glossary (opens in new window) may help you understand some terminology used.

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Questions:

  1. Why do solar and lunar eclipses occur? [Answer]
  2. So now I know that eclipses are created by the blockage of sunlight. But then why we don't have eclipses every month? The Moon completes one revolution per lunar month! [Answer]
  3. How many types of eclipses are there? [Answer]
  4. What would we observe during a lunar eclipse if we were on the Moon? [Answer]
  5. What is "eclipse year" and "eclipse season"? [Answer]
  6. Why is an eclipse year shorter than a tropical year? [Answer]
  7. What is the maximum possible number of eclipses in a calendar year? [Answer]
  8. Are there more lunar eclipses than solar eclipses? [Answer]
  9. What is the angular size of the Moon? That of the Sun? Are there more annular eclipses than total eclipses (of the sun)? [Answer]
  10. I know that seeing a total/annular eclipse is a rare event, but exactly how rare it is? [Answer]
  11. In terms of frequency, how evenly do visible eclipses spread for a particular place? [Answer]
  12. When will the next total/annular solar eclipse visit Hong Kong? [Answer]
  13. What is the theoretical maximum possible time of totality in solar eclipse? And what are the conditions for such duration to be reached? [Answer]
  14. What is the theoretical maximum possible time of annularity in solar eclipse? And what are the conditions for such duration to be reached? [Answer]
  15. Is long annularity a good thing? [Answer]

Answers:

  1. Eclipses are caused by the blockage of sunlight. In a lunar eclipse, the Moon "hides" behind the Earth and hence receives (virtually) no sunlight, and thus even when the Moon is supposed to be full we on Earth can't see the whole of it. For solar eclipse, the Moon is directly between the Sun and the Earth, and a simple experiment with a small circular ball will show you that the light emitted cannot reach your eyes. For this reason, solar eclipses can only take place on the first day of each lunar month (synodic month) and lunar eclipses near the fifteenth day of the lunar month. Technically speaking, solar eclipses should be termed "occultation of the Sun by the Moon", as the Sun does not cease to emit light during solar eclipses. [Back to top]
  2. Eclipses do not occur every month because the Moon's orbit around the Earth does not lie in the same plane as the Earth's orbit around the Sun. Specifically, the Moon's orbit around the Earth is tilted about 5 degrees with respect to the ecliptic plane (i.e. the plane of Earth's motion around the Sun). Therefore, when the Moon is "between" the Earth and the Sun (and also when the Earth is "between" the Sun and the Moon), the three objects are not usually collinear. In such cases eclipses will not be possible. Only when the Earth-Moon-Sun system is collinear is an eclipse possible. [Back to top]
  3. We discuss solar and lunar eclipses separately:
    • Solar eclipse - There are four types of solar eclipses:
      1. Total Eclipse: This occurs when the Moon completely covers the solar disk. Places experiencing a total solar eclipse will be significantly dimmer than the otherwise bright sky.
      2. Annular Eclipse: There are instances when the Moon looks smaller than the Sun, and even if it manages to attract people in the day by blocking the Sun it is impossible for the Moon to fully cover our star. In this case a ring will surround the dark (lunar) disk, which is called an "annulus". Annular eclipses are naturally less appealing than its total counterpart, but still the sky can be so dimmed that people not aware of any astronomical phenomenon will be able to tell something unusual has occurred.
      3. Partial Eclipse. This occurs when the Moon cannot cover the Sun nor is it inside the solar disk, which takes place at places where the Moon's penumbra strikes Earth. A crescent Sun results. An eclipse can be partial in nature, while in some other cases the eclipse may be total or annular; only that the observer is off the track of totality/annularity.
      4. Hybrid Eclipse: When the Moon is almost large enough to cover the Sun, the curvature of the Earth may allow it to do so only during some parts of a solar eclipse, when the Moon-Earth distance is at the minimum. On Earth, the phenomenon is that some places see an annular eclipse that is very close to being total and some other places see a very short total solar eclipse. Such an eclipse is called "hybrid" or "annular/total".
    • Lunar eclipse - There are three types of lunar eclipses:
      1. Total Eclipse: When the Moon fully enters the umbra of the Earth, it virtually cannot receive any light from the Sun, and we say a total eclipse has occurred.
      2. Partial Eclipse: This takes place when the Moon is only partially inside the Earth's umbra.
      3. Penumbral Eclipse: In some cases the Moon does not touch the umbra at all during the eclipse, and it only stays within the penumbra. Such event is called penumbral eclipse and is usually quite difficult to be detected by naked eyes. Actually we can subcategorize penumbral eclipses into total penumbral eclipses, one occurs when the Moon is completely inside the penumbra but is outside the umbra; and partial penumbral eclipses, when the Moon is only partially inside the penumbra. In terms of frequency partial penumbral eclipses occur much more frequently than total penumbral eclipses since the width of the penumbra is about the same as that of the Moon itself.

    Note that there is no annular eclipse of the Moon as it is always smaller than the Earth's umbral shadow.

    The various types of eclipses are summarized in the following figure:


    [Back to top]

  4. We have the following sets of equivalent observations:
    • Total lunar eclipse on Earth: Equivalent to total solar eclipse on the whole Moon;
    • Partial lunar eclipse on Earth: Some places on the Moon will see total eclipse of the Sun and partial elsewhere;
    • Total (lunar) penumbral eclipse on Earth: Everywhere on Moon will experience a partial eclipse of the Sun.
    • Partial (lunar) penumbral eclipse on Earth: Some places on the Moon will observe partial eclipse of the Sun and no eclipse elsewhere. [Back to top]
  5. If you have read the answer for the question on why eclipses do not occur every month, you should recognize that the Moon's orbit is tilted about 5 degrees with respect to the ecliptic. That means that in a lunar month, the plane that contains the path that the Moon traces is tilted by the same number of degrees (again with respect to the ecliptic). These two planes have two points of intersection, which we call "nodes" (see figure below). The Sun revolves once along the ecliptic per year, while the Moon revolves once per lunar month. An eclipse is only possible when the Sun is near the nodes, when there is a chance that it is met by the Moon. The time required for the Sun to travel along the ecliptic between subsequent lunar ascending node passages is called the length of an eclipse year, which lasts 346.62 days. Within this period, the Sun is within a narrow angle from the nodes twice (once ascending; once descending), each about 173 days apart. "Eclipse season" is the period in which the Sun is near the nodes so that eclipses are possible. Each eclipse season lasts for about 30 to 37 days, in which an eclipse of some sort must occur if the Moon happens to be near the node during this period. From this we can infer that at least one eclipse must occur in each eclipse season because the Moon does not need 30 days to revolve once and go back to its ascending node. This argument is valid for both solar and lunar eclipses.


    [Back to top]
  6. From a previous question we know that an eclipse year is only 346.62 days long, about 18.6 days shorter than a tropical year. The reason is that the nodes are not fixed in space; they actually regress at a rate of about 19 degrees per year. This reduces the time needed for the Sun to travel "from node to node", and makes the eclipse year about 5% shorter than a tropical year. [Back to top]
  7. There can be at most 7 eclipses per year if penumbral eclipses are excluded, 5 being solar and 2 lunar (OR 4 solar and 3 lunar). Recall from the question on eclipse season that each eclipse year lasts 346.62 days, and in each eclipse season there can be at most 2 eclipses (One eclipse season is 30 to 37 days long while the Moon requires less than 30 days between ascending node passages). That means if the first eclipse season of a calendar year occurs early, there can actually be three distinct periods that the Sun is close to the nodes. Hence the maximum number of any type of eclipse in a given year is 5 (= 1 + 2 + 2). As a piece of additional information, if there are 5 solar eclipses in a calendar year, 4 of them must be partial. In some sense, you need to sacrifice eclipse quality for eclipse quantity. [Back to top]
  8. Contrary to popular belief, there are actually more solar eclipses than lunar eclipses. If we do not count penumbral lunar eclipses (in which the moon only enters the Earth's penumbra, a phenomenon usually not visible to the naked eyes), the ratio of solar to lunar eclipses is about 3:2. However if we takes penumbral lunar eclipses into account, the ratio will be close to 1:1, with solar eclipses still outnumbering lunar eclipses. The reason why people have the misconception that lunar eclipses occur way more frequently than solar eclipses is due to their difference in visibility - for lunar eclipses anyone who has a clear night view can enjoy the process, and with the continuously-rotating Earth, slightly more than half of the globe will have the opportunity to observe any single lunar eclipse. However, in solar eclipses the path of totality or annularity is extremely narrow, and if you are outside that path, chances are that you only observe a partial eclipse that is hardly discernible (the eclipse will imitate no more than a cloudy sky even when the magnitude is 0.6 to 0.7 - only when more than 80% of the sun is eclipsed you can notice a appreciably different sky). Hence the fact is that you observe more lunar eclipses than solar eclipses in terms of proportions and excitement, and this has nothing to do in terms of eclipse occurrences (when you can't see something, it doesn't mean that it hasn't occurred). [Back to top]
  9. There are more annular eclipses than total (solar) eclipses. This is because on the average, the Moon is not large enough to cover the whole Sun. The table below lists the angular sizes:

      Maximum Mean Minimum
    Angular size (diameter) of the Sun 32' 32'' 31' 59" 31' 28''
    Angular size (diameter) of the Moon 33' 32'' 31' 05" 29' 23"

    Therefore, the Moon has the capacity of occulting the whole Sun, but this does not happen very frequently. There is some evidence that the Moon is receding from the Earth (at a rate of about 3.8 cm per year), which causes the angular size of the Moon to decrease. However the speed of recession is so slow that it takes more than 600 million years from now for the Moon's angular size to be always smaller than that of the Sun (and hence total solar eclipse is impossible). So you can possibly throw away this piece of fact; there is no way you need to worry that you will not be able to see total eclipses due to lunar recession.

    The fact that the Sun and the Moon appear roughly the same size is a nice coincidence. This is because the mean Sun-Earth distance is about 390 times longer than the Moon-Earth distance, while the solar diameter is about 400 times that of the Moon. These two numbers are very close to each other. [Back to top]
  10. In fact it is extremely rare - so rare that you may not have the opportunity to see it once in your lifetime if you don't travel (hence you always learn from the media that a large crowd gathers whenever a total solar eclipse occurs). Celestial mechanics expert Jean Meeus has calculated (statistically, using eclipses from 1700 CE to 2299 CE) that for any given place on Earth, a total solar eclipse occurs once every 375 years while an annular one occurs every 224 years. Subdividing across different latitudes, he obtained the following result:



    For example, at 20°N, one will, on average, need to wait for 364 years to see a total solar eclipse and 274 years to see an annular solar eclipse. [Back to top]
  11. Due to the complicated interactions between celestial motions, it is difficult, if not impossible, to find a fixed trend for a place regarding the frequency of eclipses. Take total solar eclipses as examples - the continental United States was last visited by the path of totality on 26 February, 1979. However this pretty big area will not be visited by any total eclipses until 21 August, 2017, almost 38.5 years after the 1979 visit. However in just about 6.5 years' time, totality will visit the region again on 8 April, 2024. Hence, accurate yet tedious computations are needed to confirm solar eclipse visibility, and simple extrapolation will fail for sure. (However, there exists certain patterns for eclipse occurrences, and it may be possible to roughly estimate the nature and viewability using such eclipse cycles. The 18-year-and-11-day Saros cycle is one of these patterns discovered.) [Back to top]

    Eclipses mentioned above: (Left) 1979/02/26; (Middle) 2017/08/21; (Right) 2024/04/08. Click to view at their original sizes
  12. The last annular eclipse visible in Hong Kong occurred on 21 May, 2012, although the cloudy skies that morning means that no one was able to observe annularity. The next annular solar eclipse in Hong Kong will take place on 9 May, 2320, while the next total solar eclipse here will occur on 21 March, 2881. This page lists all total and annular solar eclipses visible in Hong Kong between 1501 and 3000 CE. [Back to top]
  13. The current theoretical maximum is 7 minutes and 32 seconds at the point of maximum eclipse, and is slowly changing because of celestial perturbations. To create such a long eclipse the following conditions are needed:
    • The solar eclipse should occur in the (northern hemisphere) summer months, because then the Sun is furthest away from the Earth and will appear smaller on the sky. (Seasons have nothing to do with this very small difference in the Sun-Earth distance within a year.)
    • The moon should be near its perigee (closest to Earth), so that it appears as large as possible.
    • The location of greatest eclipse should be close to the equator (this is related to the concept of "gamma" distance, which is more technical and will appear in later questions). This is because the Moon, likes the Earth, moves from west to east. Totality is so short because the Moon's orbital speed around the Earth is not slow, and that as seen from Earth the lunar disk moves rapidly across the Sun. By situating near the equator the Earth's rotation speed will partially offset the speed at which the lunar shadow proceeds on Earth, and will lengthen the duration of totality. [Back to top]
  14. The current theoretical maximum is 12 minutes and 30 seconds at the point of maximum eclipse. In this case the optimal conditions are:
    • The solar eclipse should occur in the (northern hemisphere) winter months, because then the Sun is closest to the Earth and will appear larger on the sky.
    • The moon should be near its apogee (farthest away from Earth), so that it appears as small as possible.
    • The location of greatest eclipse should be close to the equator. The reason is the same as that for total eclipses. [Back to top]
  15. Yes and No. It depends on what do you want to get during an annular eclipse. If you want to achieve an annular eclipse as long as possible, of course this will serve your need. But what is more important is that long annularity implies that the angular size of the Moon is significantly smaller than that of the Sun, and consequently the masked area is smaller. This means that the change in ambient environment will not be that dramatic compared with that when the Moon's angular size is larger.

    Nevertheless, in a long annular eclipse the eclipse path is usually wider because the lunar disk can be accommodated within the solar disk more easily. That, in some sense, brings enjoyment to more people and can be considered as a benefit of long annular eclipses. [Back to top]

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Last Accessed 最近訪問日期: Thu Aug 24 2017 14:51:50 HKT
Last Modified 最近修訂日期: Mon Mar 07 2016

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