Mercury at greatest elongation west

Dominic Ford, Editor
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The sky at

Mercury will be well placed for observation in the dawn sky, shining brightly at mag -0.5.

From Fairfield, it will be difficult to observe as it will appear no higher than 12° above the horizon. It will rise at 05:22 (EST) – 1 hour and 49 minutes before the Sun – and reach an altitude of 12° above the south-eastern horizon before fading from view as dawn breaks at around 06:44.

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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 21° to the Sun's west.

Mercury in coming weeks

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

27 Nov 2018 04:09 EST – Mercury at inferior solar conjunction
11 Dec 2018 07:31 EST – Mercury at dichotomy
15 Dec 2018 10:17 EST – Mercury at greatest elongation west

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 before sunrise Mercury will rise each night; all times are given in Fairfield local time.

Date Sun
sets at
rises at
Altitude of Mercury
at sunrise
Direction of Mercury
at sunrise
08 Dec 201807:0405:2415°west
15 Dec 201807:0905:2216°west
22 Dec 201807:1405:3814°west
29 Dec 201807:1606:0011°west
05 Jan 201907:1706:22west
12 Jan 201907:1606:43south-west
19 Jan 201907:1307:00south-west
26 Jan 201907:0807:13-1°south-west
02 Feb 201907:0207:21-4°south-west
09 Feb 201906:5407:25-6°south-west
16 Feb 201906:4507:24-8°south-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 16h02m10s -18°24' Scorpius -0.5 6.6"
Sun 17h30m -23°16' Ophiuchus -26.7 32'30"

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 rises a few hours ahead of the Sun, the altitude it reaches Mercury above the horizon before sunrise 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 Fairfield varies between 72° (sunrise at the autumn equinox) and 25° (sunrise at the spring equinox). On December 15, the ecliptic is inclined at 47° to the eastern dawn 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 Fairfield.

The sky on 15 December 2018
Twilight ends
Twilight begins

8-day old moon
Waxing Gibbous


8 days old

Rise Culm. Set
Mercury 05:22 10:18 15:14
Venus 03:22 08:44 14:06
Moon 12:33 18:18 00:03
Mars 11:50 17:35 23:20
Jupiter 05:56 10:41 15:26
Saturn 08:17 12:55 17:33
All times shown in EST.


The circumstances of this event were computed using the DE405 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.

Related news

15 Dec 2018, 10:17 EST  –  Mercury at greatest elongation west
26 Feb 2019, 16:29 EST  –  Mercury at greatest elongation east
11 Apr 2019, 11:37 EDT  –  Mercury at greatest elongation west
23 Jun 2019, 20:48 EDT  –  Mercury at greatest elongation east

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