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A total lunar eclipse, during which the Moon would take an uncommon coppery-red hue, will be visible across the territory in Hong Kong on 8 November 2022.

The eclipse has already begun when the Moon rises at 5:37 pm that evening. At 6:16 pm, the total phase begins with the Moon entering the Earth's umbra completely and will last about 1 hour and 26 minutes. The subsequent partial phase will end at 8:49 pm. The next total lunar eclipse visible in Hong Kong will occur on 8 September 2025.

Interestingly, the lunar eclipse of 8 November will be accompanied by another astronomical event, the lunar occultation of Uranus. Since the planet Uranus is a lot dimmer than the Moon, under most circumstances even if they are close together, the former is often hidden in the latter's glare. Only in a total lunar eclipse when the brightness of the Moon drops significantly, we can see the Moon and Uranus side by side in a telescope. During this total eclipse, the Moon will pass in front of Uranus, resulting in an occultation event of the planet.

Stage Hong Kong Time Moon's Altitude (degrees) Moon's Azimuth (degrees)
Moon enters penumbras (P1) Not visible in Hong Kong
Moon enters umbra (U1)
Moonrise 17:37 - 72
Total eclipse begins(U2) 18:16 8 75
Occultation begins 18:58 17 79
Maximum eclipse(M) 18:59 17 79
Total eclipse ends(U3) 19:42 27 82
Occultation ends 19:47 28 82
Moon exits umbra(U4) 20:49 42 87
Moon exits penumbras(P4) 21:58 58 93

*For more about occultation, please check the section "Lunar Occultation of Uranus".

Eclipse Magnitude: 1.36
Duration of total eclipse: 1 hour and 26 minutes
Observable area: East Asia, Pacific Ocean, North America, Oceania

The above data of lunar eclipse is based on "The Astronomical Almanac" jointly published by the UK Hydrographic Office and the United States Naval Observatory. Further computer calculation that reflects observation details in Hong Kong are listed. The occultation time is generated by professional astronomical software.

The position of Uranus during the lunar occultation. The occultation starts at the East side of the Moon and ends near the South side of the Moon. In fact, the position change of Uranus is mainly due to the motion of the Moon. In this image, the Moon is fixed in order to show the relative motion of the Uranus. The size of Uranus is not drawn to scale in the above diagram.

As the Moon orbits the Earth, the position of the Moon on the celestial sphere shifts from West to East gradually (therefore the rising time of the Moon always delay day by day), and since the orbital plane of the Moon is very close to the orbital plane of the planets, the Moon will sometimes get close to the planets visually. When the Moon is close enough to a planet so that it blocks the light from the planet, a lunar occultation event occurs. From many lunar occultation events, the lunar occultation of Venus draws most attention as these two objects are very easy to be observed since they are both very bright in the sky.

In comparison, the brightness of Uranus is much lower, which is near the limit of human eye. Telescope is almost indispensable in order to observe it. However, when the Moon is near Uranus, even with a telescope, the bright moonlight will make the faint and tiny Uranus hardly observable. Yet, when a total lunar eclipse occurs, the brightness of the Moon drops significantly. At that time, the observation of Uranus becomes much easier.

At this time when the lunar occultation of Uranus occurs, the angular diameter of Uranus is about 3.7 arcseconds, which is about 1/500 of that of the Moon. Its apparent magnitude is about 5.7. Using a telescope with high magnification, Uranus will appear as a tiny blue disc where its edge will look relatively darker. When occultation starts, it will take about 9 seconds for Uranus to disappear completely against the limb of the Moon; when occultation ends, it will take roughly the same time for Uranus to reappear completely against the other limb of the Moon. The entire occultation event will last for about 49 minutes. If a telescope with low magnification is used, Uranus will appear almost as same as an ordinary star, but observers can see the change in brightness of Uranus within a few seconds during the beginning and the end of occultation. Such change is not observable in other star-related lunar occultation events.

Apparent Magnitude of Uranus: 5.7
Duration of lunar occultation of Uranus: about 49 minutes

Predicted time of disappearance  18:57:55 – 18:58:04
Predicted time of reappearance  19:47:07 – 19:47:16

The above predictions are based at the Hong Kong Space Museum, Tsim Sha Tsui, Kowloon, Hong Kong. The occultation time varies by a few tens of seconds across Hong Kong.

How to observe?

The Moon will rise in the east-northeast and is still near the horizon when the total eclipse begins. You should find a site with an unobstructed view, such as the Central and Western District Promenade, the Avenue of Stars in Tsim Sha Tsui or the Pak Shek Kok Promenade, to observe the event.  At about 7 pm when the sky is dark enough with the Moon at a higher altitude, the lunar eclipse can be seen with relative ease across the territory, if weather permits.  As for Uranus, as its brightness is near the limit of naked eye visibility, a telescope is required to observe it clearly.  When observed from the ground, Uranus will enter the Moon from its bottom (east) and emerge from its right (south).

Lunar eclipse occurs when the Moon contacts the Earth's shadow. However, as the edge of the Earth's shadow is blurry, it is very difficult to predict and verify the start time and end time of a lunar eclipse. Back in the 18th century, astronomers realised that the Earth's shadow had to be enlarged by around 1.5% to 2.5% in order to match the actual observed time of the lunar eclipses. Furthermore, the enlargement factor varies for every lunar eclipses. In 19th century, William Chauvenet, astronomer of the United States Navy, chose to enlarge the Earth's shadow by 2% for all his lunar eclipse predictions. His method is still being used by the United States Naval Observatory to date. French astronomer André-Louis Danjon developed another method in mid-20th century. He added a notional eclipse-forming layer of 75km height onto the Earth in his predictions. These two methods are commonly used in predicting times of lunar eclipses. However, both methods assume a circular Earth's shadow, and the radius of Earth is taken from the average of that at 45 degrees latitude. Yet, it is difficult to verify the accuracy of these methods from observation as the boundary of the Earth's shadow is unclear.

In 2014, David Herald and Roger W. Sinnott analysed 22,539 observations made at 94 lunar eclipses from 1842 to 2011, and established a new method to predict lunar eclipses. They consider the Earth as an oblate spheroid, and take into account the position of the Sun, the orientation of the shadow of the oblate Earth during a lunar eclipse can be obtained. Furthermore, based on Danjon's method, they added a notional eclipse-forming layer of 87km height onto the Earth. Some recent publications on lunar eclipse prediction adopted this method. This method can be regarded as the most advanced one at present. Below shows the time predictions of this lunar eclipse based on different methods:

Stage

Time (HKT), ΔT=69.2s

Chauvenet

Danjon

Herald & Sinnott

Total eclipse begins

18:16:21

18:16:43

18:16:14

Maximum eclipse

18:59:13

18:59:13

18:59:13

Total eclipse ends

19:42:02

19:41:41

19:41:53

Moon exits umber

20:49:26

20:49:07

20:49:25

Moon exits penumbra

21:57:52

21:56:14

21:56:33

        

Magnitude

1.364

1.359

1.361

The positions of the Sun and the Moon are calculated based on NASA's JPL DE440 ephemeris. As the speed of Earth's rotation is not a constant, ΔT has to be added in order to convert the time data obtained from the calculation into the universal time or Hong Kong time (UTC+8). The value of ΔT can only be obtained via observation. While predicting astronomical phenomena, ΔT is also predicted based on its trend. Some common astronomical forecast data may differ by a few seconds for the same event because they have chosen a different value of ΔT.

A total lunar eclipse, during which the Moon would take an uncommon coppery-red hue, will be visible across the territory in Hong Kong on 8 November, 2022. At the same time during the lunar eclipse, another astronomical event, lunar occultation of Uranus, will take place. The Hong Kong Space Museum will arrange two live streaming programmes to livecast the events as well as introducing the features of the event and providing some observation tips.

Topic: Lunar Eclipse: What is it?
Date: 5.11.2022 (Saturday)
Time: 3:00pm – 4:00pm

Topic: Total Lunar Eclipse cum Lunar Occultation of Uranus
Date: 8.11.2022 (Tuesday)
Time: 6:00pm – 9:00pm

Speaker: Mr. CHAN Chun-lam (Assistant Curator, Hong Kong Space Museum), Ms. CHEUNG Wing-shan (Assistant Curator, Hong Kong Space Museum)
Platform: YouTube channel of the Hong Kong Space Museum
https://www.youtube.com/hkspacem

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