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Lunar Eclipse

The cause of Lunar Eclipse

The earth throws a long shadow behind the side facing the sun. A lunar eclipse occurs when the moon enters the earth's shadow. This shadow has two parts: the total shadow called the umbra and the partial shadow termed the penumbra. (See Fig. 1)

If the moon becomes completely immersed into the umbra, a total eclipse occurs; and if only a portion of it falls into the umbra, a partial eclipse occurs.

During totality, the moon is not completely invisible. This is due to the fact that some sunlight are refracted by the earth's thin atmosphere into the umbra and illuminate the moon, making it coppery-red in colour. Its brightness varies for different total eclipses depending on the geometry of the moon's path through the umbra and also on the earth's atmospheric conditions.

Sometimes the moon only enters the penumbra without touching the umbra. This is a penumbral lunar eclipse. During a penumbral lunar eclipse, only the apparent brightness of the moon will become dimmer but the limb of the moon will not be obscured by the earth's shadow.

Picture shows how the lunar eclipse happen

Figure 1

Different Stages in Lunar Eclipse

There are 7 stages during the occurrence of an eclipse:

  1. "Moon enters penumbra" - the first contact of the moon and the earth's penumbra
  2. "Moon enters umbra" - when the moon enters the earth's umbra, this is the start of partial lunar eclipse
  3. "Total eclipse begins" - when the moon touches the umbra internally, at this time, the moon is totally immersed in the umbra and total eclipse starts
  4. "Middle of eclipse" - occurs when the centre of the moon is closest to the centre of the earth's umbra
  5. "Total eclipse ends" - when the moon touches the umbra internally again, this signifies the end of total eclipse
  6. "Moon leaves umbra" - the time when the moon touches the umbra externally, this is the end of partial lunar eclipse
  7. "Moon leaves penumbra" - when the moons leaves the penumbra

During a partial eclipse, the "total eclipse begins" and "total eclipse ends" would not occur. In a penumbral eclipse, there would only be "moon enters penumbra", "middle of eclipse" and "moon leaves penumbra".

It shows difference states of eclipse

Figure 2 The different stages during a lunar eclipse


Past Lunar Eclipses visible in Hong Kong

Date Observation in Hong Kong
15 April 1995 Partial Lunar Eclipse
8-9 October 1995 Penumbral Lunar Eclipse
4 April 1996 Total Lunar Eclipse
17 September 1997 Total Lunar Eclipse
6 September 1998 Penumbral Lunar Eclipse
31 January 1999 Penumbral Lunar Eclipse
28 July 1999 Partial Lunar Eclipse
16 July 2000 Total Lunar Eclipse
10 January 2001 Total Lunar Eclipse
5 July 2001 Partial Lunar Eclipse
30 December 2001 Penumbral Lunar Eclipse
26 May 2002 Penumbral Lunar Eclipse
25 June 2002 Penumbral Lunar Eclipse
9 November 2003 Penumbral Lunar Eclipse
5 May 2004 Total Lunar Eclipse
24 April 2005 Penumbral Lunar Eclipse
17 October 2005 Partial Lunar Eclipse
15 March 2006 Penumbral Lunar Eclipse
8 September 2006 Partial Lunar Eclipse
4 March 2007 Total Lunar Eclipse
28 August 2007 Total Lunar Eclipse

17 August 2008

 Partial Eclipse

9 February 2009

 Penumbral Lunar Eclipse

Next Lunar Eclipse visible in Hong Kong

Date Type of Eclipse Observation in Hong Kong

1 January 2010

 Partial Lunar Eclipse Partial Lunar Eclipse