Stargazers and amateur astronomers will have a treat on the night of 27 July. A Mars perihelic‡ opposition will occur followed by a total lunar eclipse in the early morning of the next day. The two astronomical events will be visible if weather permits.
During Mars opposition, Mars, the Earth and the Sun lie on a straight line, with Mars and the Sun located exactly on the opposite sides of the Earth. When Mars is at its perihelion during the opposition, it is called the Mars perihelic opposition (Figure 1). During the event, Mars is also closest to the Earth, and therefore the planet will look brighter than usual. Mars perihelic opposition occurs every 15 or 17 years.
The last Mars perihelic opposition took place in 2003, and members of the public may have the chance to witness the event again with the naked eye on 27 July. Mars will appear as a bright reddish orange dot in the nights around the opposition, with its brightness second only to the Moon and Venus in the night sky. Details of the coming Mars perihelic opposition are as follows:
| Date | Hong Kong Time | Phenomenon | Elevation | Direction (Azimuth) |
| July 27 (Friday) |
1.13pm | Mars opposition | Below the horizon | — |
| 7.06pm | Sunset | -1 degree | West-northwest (291 degrees) |
|
| 7.18pm | Mars rise | -1 degree | East-southeast (117 degrees) | |
| July 28 (Saturday) |
0.34am | Mars transit | 42 degrees | South (180 degrees) |
| 5.51am | Mars set | -1 degree | West-southwest (242 degrees) |
Members of the public may also observe the second total lunar eclipse of the year in the early hours of 28 July. The eclipse will begin at 1:13 am, with the stage of total eclipse lasting for 1 hour and 44 minutes, the longest duration since 2000. The event will be visible at most places with an unobstructed view to the southwest till moonset at 6:00 am (Figures 2 and 3). Details of the total lunar eclipse are as follows:
| Date | Hong Kong Time | Phenomenon | Elevation | Direction (Azimuth) |
| July 27 (Friday) |
6.45pm | Moonrise | -1 degree | East-southeast (111 degrees) |
| July 28 (Saturday) |
1.13am | Moon enters penumbra | 46 degrees | South-southwest (197 degrees) |
| 2.24am | Moon enters umbra | 39 degrees | Southwest (217 degrees) |
|
| 3.30am | Total eclipse begins | 29 degrees | Southwest (231 degrees) |
|
| 4.22am | Maximum eclipse | 19 degrees | West-southwest (239 degrees) |
|
| 5.14am | Total eclipse ends | 9 degrees | West-southwest (245 degrees) |
|
| 5.53am | Sunrise | -1 degree | East-northeast (69 degrees) |
|
| 6.00am | Moonset | -1 degree | West-southwest (250 degrees) |
|
| 6.19am | Moon leaves umbra | Below the horizon | — | |
| 7.30am | Moon leaves penumbra | Below the horizon | — |
Members of the public can watch the whole event via a webcast to be jointly provided by the Hong Kong Observatory, the Hong Kong Space Museum, the Ho Koon Nature Education cum Astronomical Centre, the Po Leung Kuk Ngan Po Ling College and the Hong Kong Sheng Kung Hui Solar Tower‧Camp on the following webpage:
www.hko.gov.hk/gts/event/webcast-20180728.htm.
For the weather on July 27 and 28, please refer to the latest nine-day weather forecast issued by the Hong Kong Observatory at the following webpage:
www.hko.gov.hk/wxinfo/currwx/fnd.htm.
As regards the astronomical observation conditions for that night, please refer to the “Weather Information for Astronomical Observation” webpage:
www.hko.gov.hk/gts/astronomy/astro_portal.html.
The next lunar eclipse visible in Hong Kong will occur on 17 July 2019, and will be a partial lunar eclipse.
Note ‡: Perihelion is the point in the planet’s orbit when it is closest to the Sun.
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