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BAA Observing calendar

Mercury at greatest elongation east

Monday, 2019, June 24 - 01:48

In the southern hemisphere Mercury will be well placed for observation in the evening sky, shining brightly at mag 0.4.

From London however, it will not be observable – it will reach its highest point in the sky during daytime and will be no higher than 4° above the horizon at dusk.

Mercury's orbit lies closer to the Sun than the Earth's, meaning that it always appears close to the Sun and is very difficult to observe most of the time.

It is observable only for a few days each time it reaches greatest separation from the Sun – moments referred to as greatest elongation.

These apparitions take place alternately in the morning and evening skies, depending whether Mercury lies to the east of the Sun or to the west.

When it lies to the east, it rises and sets a short time after the Sun and is visible in early evening twilight. When it lies to the west of the Sun, it rises and sets a short time before the Sun and is visible shortly before sunrise.

On this occasion, it lies 25° to the Sun's east.

Mercury in coming weeks

The key moments in this apparition of Mercury are as follows:

18 Jun 2019 03:51 BST  – Mercury at dichotomy
24 Jun 2019 01:48 BST  – Mercury at greatest elongation east
21 Jul 2019 13:28 BST  – Mercury at inferior solar conjunction

After greatest elongation, the distance between Mercury and the Sun will decrease each night as it sinks back into the Sun's glare. The table below lists how long Mercury will remain up after sunset each night; all times are given in London local time.

Date Sun
sets at
Mercury
sets at
Altitude of Mercury
at sunset
Direction of Mercury
at sunset
17 Jun 2019 21:14 23:02 13° west
24 Jun 2019 21:16 22:50 12° west
01 Jul 2019 21:16 22:26 west
08 Jul 2019 21:13 21:50 west
15 Jul 2019 21:07 21:06 north-west
22 Jul 2019 20:59 20:22 -5° north-west
29 Jul 2019 20:50 19:48 -8° north-west
05 Aug 2019 20:39 19:34 -8° north-west
12 Aug 2019 20:26 19:36 -6° north-west
19 Aug 2019 20:12 19:47 -4° north-west
26 Aug 2019 19:58 19:53 -1° north-west

A graph of the angular separation of Mercury from the Sun around the time of greatest elongation is available here.

Mercury's position

The position of Mercury when it reaches greatest elongation will be:

Object Right Ascension Declination Constellation Magnitude Angular Size
Mercury 07h57m10s +21°16' Gemini 0.4 8.2"
Sun 06h08m +23°25' Gemini -26.7 31'28"

The coordinates above are given in J2000.0.

Seasonal effects

At each apparition, Mercury reaches a similar separation from the Sun – around 18–28°. This distance is set by the geometry of how big Mercury's orbit is, and how far away it is from the Earth.

Nonetheless, some times of the year are more favourable for viewing Mercury than others.

It appears most favourably in the evening sky around the time of the local spring equinox, and most favourably in the morning sky around the local autumn equinox.

These dates are reversed in the northern and southern hemispheres, such that a good apparition in one hemisphere will not be easily observable from the other.

This is comes about because Mercury always lies close to the line of the ecliptic, shown in yellow in the planetarium above. This is the line through the zodiacal constellations that the Sun follows through the year, and marks the flat plane in space in which all of the planets circle the Sun.

When Mercury remains in the sky for a few hours after the Sun has set, its altitude above the horizon depends on two factors.

One is its angular separation from the Sun. But equally important is how steeply the line of the ecliptic is inclined to the horizon.

If Mercury is widely separated from the Sun along the ecliptic, this may not translate into a high altitude if the ecliptic meets the horizon at a very shallow angle, running almost parallel to it.

Conversely, if the ecliptic is almost perpendicular to the horizon, a much smaller separation from the Sun may place Mercury higher in the sky.

The inclination of the ecliptic plane to the horizon at London varies between 61° (sunset at the spring equinox) and 15° (sunset at the autumn equinox). On June 24, the ecliptic is inclined at 30° to the western sunset horizon, as shown by the yellow line in the planetarium view above, meaning that on this occasion Mercury is not ideally placed for viewing from London.

This entry in the observing calendar was provided by In-The-Sky.org

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