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Eclipse Q&A - Page 3
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Questions (continued):

  1. What are the safest ways of observing solar eclipses? [Answer]
  2. Are overexposed negatives, buckets of ink and sunglasses safe for viewing solar eclipse? [Answer]
  3. Are ocular precautionary measures necessary for a lunar eclipse? [Answer]
  4. Across different centuries, how are the number of tetrads (4 total lunar eclipses in a row) and total penumbral (lunar) eclipses related, and how do their frequencies change? [Answer]
  5. How is the general shape of the corona during a total solar eclipse related to the solar (sunspot) cycle? [Answer]
  6. What are "geocentric conjunction" and "greatest eclipse"? [Answer]
  7. During solar eclipses, sometimes the Moon tracks an irregular path relative to the Sun. Why is this so? [Answer]


  1. Safe ways include:

    • Pinhole projection method: Punch a tiny hole on one cardboard, and hold it in front of another. Let sunrays pass through that hole and observe the projected image on the other cardboard with your back facing the Sun. Adjust the distance between the cardboards to obtain a sharp image. This method can also be applied when you don't have a cardboard on hand. Use something with small holes and try to focus light through those holes onto a bright surface; tiny eclipsed shapes will be projected. Small holes on leaves and images from binoculars (but NOT watching through them) are very good examples.
    • Watching through solar filters: Solar filters can be purchased at a relatively low cost from stores specialized in optical instruments. These filters will get rid of almost all harmful rays and will allow comfortable viewing of the Sun. Number 14 Welder's glass will also allow safe viewing.

    If one desires to observe a larger image, suitable filters must be installed to the telescopes or binoculars before observation. Also note that while it is safe to observe the fully eclipsed Sun directly, it is not when the Sun is partially eclipsed even when only a few Bailey's beads remain. [Back to top]

  2. Fully exposed true black-and-white films contain silver that makes them safe. Do not use exposed colour films as they offer virtually no protection. Buckets of ink were used in the old days because technology was not advanced enough for them to create better filters; this method is actually not safe for continuous viewing. Sunglasses offer you a dimmer environment on a sunny day, but they do not have the capacity to filter harmful rays. In short, all three instruments mentioned in this question are not suggested. [Back to top]
  3. The answer is no. Just ask yourself whether protective instruments are needed to see the full Moon every month - the reflected light from the Moon is much dimmer than the sunrays themselves and poses no danger to the eye. [Back to top]
  4. The number of tetrads is positively related to the number of total penumbral eclipses in a century. Across centuries, three consecutive centries of high tetrad (and also total penumbral eclipse) occurrences are followed by two or three centuries of very low number of occurrences. See the table below:

    Century # Total Penumbral Eclipse # Tetrad
    6th (i.e. 501 - 600 CE) 1 0
    7th 2 0
    8th 2 3
    9th 5 8
    10th 3 6
    11th 3 0
    12th 0 0
    13th 1 0
    14th 8 6
    15th 4 4
    16th 2 6
    17th 2 0
    18th 0 0
    19th 2 0
    20th 9 5
    21st 5 7
    22nd 5 4
    23rd 1 0
    24th 0 0
    25th 6 4

    [Back to top]
  5. The corona is more circular at sunspot maximum, and is bulged at the equator at sunspot minimum. This is related to the solar activities at that time - at sunspot maximum, there are generally more active solar activities and the magnetic field is twisted to a maximum, leading to more pronounced corona from all angles.

    Each solar cycle lasts for about 11 years. For example, solar activity was at a maximum in 2000 and minimum in 2008, and thus we can predict that a bulged corona could be observed during the total solar eclipse in 2009.

    Image of the Sun taken by a coronagraph on 21 June, 2009, at a time of solar minimum. Note that the corona is more concentrated near the equator. [Back to top]
  6. "Geocentric conjunction" occurs when the right ascensions of the Sun and that of the Moon coincide as seen from the Earth's centre, while "Maximum eclipse" occurs when the magnitude of the eclipse is the greatest. These two do not necessarily happen at the same time because the Sun is not necessarily exactly at the node when maximum eclipse occurs. This can be more easily understood with the help of the following diagram (not to scale):

    The same principle works for lunar eclipses, with "Sun" replaced by "antisolar point" (i.e. imaginary point opposite to the Sun as seen on the celestial sphere) - the Sun travels along the ecliptic, so does the antisolar point. [Back to top]
  7. The question statement is best illustrated by the following figure which graphs the position of the Moon relative to the Sun during a solar eclipse:

    Partial solar eclipse in Hong Kong on 20 June, 1955.

    This happens because the graph is drawn based on the horizontal coordinate system. The top of the graph corresponds to the direction right above the centred celestial body (the Sun in this case) seen by an observer. If the Sun is very close to the zenith at culmination, the direction corresponding to "the top of the Sun" with respect to the horizon can change rapidly, and so can the relative angles of other celestial bodies. Thus the lunar path about the Sun appears irregular. In general, such occurrence is more likely when the Sun's altitude is above 80°. The following animation helps to explain this situation:

    Animation corresponding to the eclipse above. Note that the Sun is centred and the camera angle is always parallel to the horizon. [Back to top]

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