Mercury at greatest elongation west

Dominic Ford, Editor
From the Inner Planets feed

Objects: Mercury
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Mercury will reach its greatest separation from the Sun in its December 2017–January 2018 morning apparition. It will be shining brightly at mag -0.4.

From Fairfield , this apparition will be reasonably placed but nonetheless tricky to observe, reaching a peak altitude of 16° above the horizon at sunrise on 29 Dec 2017.

Begin typing the name of a town near to you, and then select the town from the list of options which appear below.

The table below lists how high above the horizon Mercury will appear at sunrise over the course of the apparition. All times are given in Fairfield local time.

Date Sun
rises at
rises at
at sunrise
at sunrise
17 Dec 201707:1506:23south-east
20 Dec 201707:1705:5911°south-east
23 Dec 201707:1805:4314°south-east
26 Dec 201707:1905:3515°south-east
29 Dec 201707:2005:3316°south-east
01 Jan 201807:2105:3515°south-east
04 Jan 201807:2105:3914°south-east
07 Jan 201807:2105:4514°south-east
10 Jan 201807:2005:5213°south-east
13 Jan 201807:2005:5911°south-east
16 Jan 201807:1806:0610°south-east
19 Jan 201807:1706:14south-east
22 Jan 201807:1506:21south-east
25 Jan 201807:1306:28south-east
28 Jan 201807:1106:34south-east

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

Observing Mercury

The December 2017–January 2018 morning apparition of Mercury
27 Dec 2017 – Mercury at dichotomy
27 Dec 2017 – Mercury reaches highest point in morning sky
01 Jan 2018 – Mercury at greatest elongation west

Mercury's orbit lies closer to the Sun than the Earth's, meaning it always appears close to the Sun and is lost in the Sun's glare much of the time.

It is observable for only a few weeks each time it reaches greatest separation from the Sun – moments referred to as greatest elongation. These apparitions repeat roughly once every 3–4 months, taking 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.

However, some times of the year are more favourable for viewing Mercury than others. From Fairfield, it reaches a peak altitude of between 10° and 19° above the horizon at sunrise during each morning apparition, depending on the time of year. During its December 2017–January 2018 apparition, it will peak at 16° above the horizon at sunrise on 29 Dec 2017.

This variability over the course of the year is due to a combination of two factors.

The inclination of the ecliptic to the horizon

The inclination of the ecliptic to the horizon changes over the course of the year, affecting how high planets close to the Sun appear in the sky.

At all times, Mercury lies close to a line across the sky called the ecliptic, which is shown in yellow in the planetarium above. This line traces the path that the Sun takes through the zodiacal constellations every year, and shows the plane of the Earth's orbit around the Sun. Since all the planets circle the Sun in almost exactly the same plane, it also closely follows the planes of the orbits of the other planets, too.

When Mercury is widely separated from the Sun, it is separated from it along the line of the ecliptic. But, at different times of year, the ecliptic meets the horizon at different angles at sunrise. This would translate into Mercury being at different altitudes above the horizon, even if its separation from the Sun was constant.

If the ecliptic meets the horizon at a shallow angle, then Mercury has to be very widely separated from the Sun to appear much above the horizon. Conversely, if the ecliptic is almost perpendicular to the horizon, Mercury may appear much higher in the sky, even if it is actually much closer to the Sun.

The seasonal dependence of this is that at sunset, the ecliptic makes its steepest angle to the horizon at the spring equinox – in March in the northern hemisphere, and in September in the southern hemisphere. Conversely, it meets the horizon at its shallowest angle at the autumn equinox. Because the seasons are opposite in the northern and southern hemispheres, a good apparition of Mercury in one hemisphere will usually be badly placed in the other.

At sunrise, these dates are also inverted, so that for morning apparitions of Mercury, the ecliptic makes its steepest angle to the horizon at the autumn equinox, and its shallowest angle to the horizon at the spring equinox.

Mercury's elliptical orbit

The orbits of the planets Mercury, Venus and Earth, drawn to scale. The orbit of Mercury is significantly non-circular. Click to expand.

A secondary effect is that Mercury is unusual among the planets for having a significantly non-circular orbit, which varies in its distance from the Sun by 52% between its closest approach (perihelion, labelled P in the diagram to the right) and greatest distance (aphelion, labelled A).

This means that Mercury's separation from the Sun at greatest elongation varies, depending where it lies relative to the aphelion or perihelion points of its orbit. In mid-September and mid-March, the Earth is well placed to view the long axis of Mercury's orbit edge-on.

So, if Mercury appears in the evening sky in mid-September, or in the morning sky in mid-March, then it appears more widely separated from the Sun than usual. Specifically, at each apparition, Mercury reaches a separation from the Sun of between 18 and 28°. During its December 2017–January 2018 apparition, it will reach a maximum separation of 22° to the Sun's west at greatest elongation.

The optimum time for an apparition of Mercury

The maximum altitude of Mercury during all its morning apparitions between 2000 and 2050, as a function of the day of the year on which greatest western elongation occurs. Different colours show the altitudes observed from different latitudes. Click to expand.

The two effects described above are of similar magnitude, though the inclination of the ecliptic to the horizon is the more significant. They conspire to make Mercury much easier to observe from the southern hemisphere than from the north.

In the southern hemisphere, apparitions of Mercury which occur when the ecliptic plane is favourably inclined to the horizon also coincide with apparitions when Mercury is close to aphelion. In the northern hemisphere, unfortunately the opposite is true: when the ecliptic plane is favourably inclined, Mercury is close to perihelion.

The plot to the right shows the maximum altitude of Mercury during all its morning apparitions between 2000 and 2050, as observed from a range of different latitudes on Earth. The highest altitudes are seen exclusively from the southern hemisphere.

Mercury's position

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

Object Right Ascension Declination Constellation Magnitude Angular Size
Mercury 17h11m30s -21°06' Ophiuchus -0.4 6.6"
Sun 18h49m -22°57' Sagittarius -26.7 32'31"

The coordinates above are given in J2000.0.

The sky on 01 January 2018
Twilight ends
Twilight begins

14-day old moon
Waxing Gibbous


14 days old

Rise Culm. Set
Mercury 05:34 10:19 15:04
Venus 07:15 11:49 16:22
Moon 16:44 00:02 06:13
Mars 02:49 07:58 13:06
Jupiter 03:02 08:08 13:14
Saturn 06:35 11:14 15:52
All times shown in EST.


The circumstances of this event were computed using the DE430 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

01 Jan 2018  –  Mercury at greatest elongation west
25 Jan 2018  –  Mercury at aphelion
17 Feb 2018  –  Mercury at superior solar conjunction
10 Mar 2018  –  Mercury at perihelion

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