Mercury at greatest elongation west

2015 Feb 24

Dominic Ford – originally published on In-The-Sky.org

Across much of the world Mercury will be well placed for observation in the dawn sky, shining brightly at mag -1.9.

From London (click to change) however, it will not be observable – it will reach its highest point in the sky during daytime and will be no higher than 3° above the horizon at dawn.

Mercury in coming weeks

Over coming weeks, the distance between Mercury and the Sun will decrease each morning, and it will gradually sink back into the Sun’s glare, as the table below indicates (all times given in London local time).

Date Sun
rises at
Mercury
rises at
Altitude of Mercury
at sunrise
Direction of Mercury
at sunrise
21 Feb 2015 07:03 06:02 south-east
24 Feb 2015 06:57 06:01 south-east
27 Feb 2015 06:50 06:00 south-east
02 Mar 2015 06:44 05:59 south-east
05 Mar 2015 06:38 05:58 south-east
08 Mar 2015 06:31 05:57 south-east
11 Mar 2015 06:24 05:56 south-east
14 Mar 2015 06:17 05:54 south-east
17 Mar 2015 06:11 05:51 east
20 Mar 2015 06:04 05:48 east
23 Mar 2015 05:57 05:45 east

Mercury’s position

The exact position of Mercury when it reaches greatest elongation is as follows:

Object Right Ascension Declination Constellation Magnitude Angular Size
Mercury 20h44m40s -18°05′ Capricornus -1.9 7.0″
Sun 22h29m -09°29′ Aquarius -26.7 32’19”

The coordinates above are given in J2000.0.

The geometry of Mercury’s orbit

Mercury never ventures far from the Sun in the sky since its orbit lies closer to the Sun than the Earth’s. To see it, we are always looking inwards towards the center of the solar system, close to the Sun. Mercury is said to be at greatest enlongation when it passes either of the two points along its orbit where it appears at greatest separation from the Sun.

As it orbits the Sun, it appears alternately in the morning and evening skies. When it is to the east of the Sun, it rises and sets a short time after the Sun, becoming visible in early evening twilight. When it is to the west of the Sun, it rises and sets a short time before the Sun, becoming visible shortly before sunrise.

Seasonal variation

Each time it appears in the morning or evening sky, Mercury reaches roughly the same angular separation from the Sun – this time peaking at a distance of 26° at greatest elongation. This angle 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 from London than others.

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

This is because it always lies close to a line across the sky called the ecliptic. 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.

The altitude at which Mercury appears above the horizon at sunrise or sunset depends how steeply the line of the ecliptic is inclined to the horizon. If the plane of the ecliptic meet the horizon at a very shallow angle, Mercury will rise or set along a line which is almost parallel to the horizon, and a separation of 26° from the Sun along this line would correspond to a very low altitude in the sky.

The inclination of the ecliptic plane to the horizon at London varies between 61° (sunrise at the autumn equinox) and 15° (sunrise at the spring equinox). On February 24, the ecliptic is inclined at 15° to the eastern dawn horizon, as shown by the yellow line in the planetarium view above, meaning that on this occasion Mercury is poorly placed for viewing from London.


The details of this observing event were provided courtesy of In-The-Sky.org

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