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Cause of Lunar Eclipse

A Lunar Eclipse is a natural phenomenon. It occurs when the Earth comes in direct alignment with the Sun and the Moon, resulting in the Moon's passing into the shadow of the Earth.

Lunar Eclipse

During a Lunar Eclipse, the Sun and the Moon would travel to the opposite side in the sky (about 180° apart from each other). In other words, a Lunar Eclipse can only occur during a full moon (around the 15th of a lunar month). Why doesn't Lunar Eclipse occur every month? The answer is quite complicated. The Sun and the Moon travel around the sky in near-circular paths called the ecliptic and the Moon's path respectively. However, the Moon's path is tilted about 5° to the ecliptic, making the alignment of the Sun, the Earth and the Moon in a straight line not so frequent. Only when the Sun and the Moon are close enough to the nodes (crossing points of the ecliptic and the moon's paths) during a full moon can a lunar eclipse occur.

When the Moon totally entres the umbra (the full shadow) of the Earth, it is a Total Lunar Eclipse; When only part of the Moon passes into the umbra, it is a Partial Lunar Eclipse; As to the Penumbral Lunar Eclipse, it describes a phenomenon when the Moon only entres the penumbra (the partial or incomplete shadow) of the Earth.

As the size of the umbra of the Earth is much larger than the Moon, the Moon is always totally inside the Earth's shadow during total lunar eclipse. Hence there is no such thing as "annular lunar eclipse".

Total lunar eclipse goes through seven stages:

  1. Moon enters penumbra : Instant of first external tangency of Moon with the penumbra.
  2. Moon enters umbra : Instant of first external tangency of Moon with the umbra.
  3. Total eclipse begins : Instant of first internal tangency of Moon with the umbra.
  4. The greatest eclipse : The moment when the centre of the Moon is the closest to the centre of the Earth's shadow.
  5. Total eclipse ends : Instant of last internal tangency of the Moon with the umbra.
  6. Moon leaves umbra : Instant of last external tangency of the Moon with the umbra.
  7. Moon leaves penumbra : Instant of last external tangency of the Moon with the penumbra. It marks the end of the lunar eclipse.

For more detail, you can go to "Nature of the Universe" Chapter 7 ─ The Moon and Eclipses in Teacher's Corner.

Forthcoming lunar eclipse from 2017 to 2020:

Date 1 Type Hong Kong Hong Kong Time2 Mag.3 Moonrise Moonset
26/5/2021 Total Eclipse at Moonrise 5:44pm - 8:52pm 1.016 6:56pm (26/5) 06:08am (27/5)
19/11/2021 Partial Eclipse at Moonrise 3:18pm - 6:47pm 0.979 5:38pm (19/11) 7:12am (20/11)
16/5/2022 Total Invisible 10:27am - 13:55pm 1.420 7:12pm (16/5) 05:33am (16/5)
8/11/2022 Total Eclipse at Moonrise 5:08pm - 8:49pm 1.364 5:37pm (8/11) 7:02am (9/11)
5/5/2023 Penumbral Visible 11:12pm - 3:33am 0.989 6:32pm (5/5) 5:56am (6/5)
29/10/2023 Partial Visible 03:34am - 4:53am 0.127 5:25pm (28/10) 06:34am (9/10)
25/3/2024 Penumbral Invisible 12:51pm - 5:34pm 0.982 6:40pm (25/3) 6:18am (25/3)
18/9/2024 Partial Invisible 10:11am - 11:16am 0.091 6:37pm (18/9) 06:02am (18/9)
14/3/2025 Total Invisible 1:09pm - 4:48pm 1.183 6:36pm (14/3) 06:27am (14/3)
8/9/2025 Total Visible 0:26am - 3:56am 1.368 6:21pm (7/9) 06:21am (8/9)
3/3/2026 Total Eclipse at Moonrise 5:49pm - 9:17pm 1.156 6:22pm (3/3) 7:00am (4/3)
28/8/2026 Partial Invisible 10:33am - 1:52pm 0.935 06:49pm (28/8) 05:54am (28/8)
  1. The date showed in the table is the time for the "Moon enters umbra" or the "Moon enters penumbra".
  2. For Total / Patial lunar eclipse, the time period is for the "Moon enters umbra" to "Moon leaves embra". For Penumbral lunar eclipse, the time period is for the "Moon enters penumbra" to "Moon leaves penumbra".
  3. For Penumbral lunar eclipse, Magnitude is equal to Penumbral Magnitude.