© NASA/JPL/MESSENGER

Mercury at greatest elongation west

Tue, 12 Sep 2017 at05:11 EDT(72 days ago)
09:11 UTC

Dominic Ford, Editor
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Ashburn
The sky at

Mercury will be well placed for observation in the dawn sky, shining brightly at mag -0.4.

From Ashburn (click to change), it will be difficult to observe as it will appear no higher than 11° above the horizon. It will rise at 05:17 (EST) – 1 hour and 30 minutes before the Sun – and reach an altitude of 11° above the eastern horizon before fading from view as dawn breaks at around 06:23.

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 17° to the Sun's west.

Mercury in coming weeks

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

26 Aug 2017 16:36 EDT – Mercury at inferior solar conjunction
12 Sep 2017 05:11 EDT – Mercury at greatest elongation west
12 Sep 2017 23:34 EDT – Mercury at dichotomy
08 Oct 2017 06:14 EDT – Mercury at greatest brightness
08 Oct 2017 16:38 EDT – Mercury at superior 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 before sunrise Mercury will rise each night; all times are given in Ashburn local time.

Date Sun
sets at
Mercury
rises at
Altitude of Mercury
at sunrise
Direction of Mercury
at sunrise
05 Sep 201706:4005:3111°west
12 Sep 201706:4605:1816°west
19 Sep 201706:5305:3613°west
26 Sep 201706:5906:10west
03 Oct 201707:0606:46west
10 Oct 201707:1207:20-1°west
17 Oct 201707:1907:52-6°west
24 Oct 201707:2708:22-11°south-west
31 Oct 201707:3408:50-14°south-west
07 Nov 201706:4208:17-17°south-west
14 Nov 201706:5008:39-19°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 10h15m00s +11°14' Leo -0.4 7.2"
Sun 11h21m +04°08' Leo -26.7 31'47"

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 Ashburn varies between 74° (sunrise at the autumn equinox) and 27° (sunrise at the spring equinox). On September 12, the ecliptic is inclined at 73° to the eastern dawn horizon, as shown by the yellow line in the planetarium view above, meaning that on this occasion Mercury is very favourably placed for viewing from Ashburn.

The sky on 12 September 2017
Sunrise 06:47
Sunset 19:22
Twilight ends
20:53
Twilight begins
05:16

22-day old moon
Age of Moon
22 days

All times shown in EDT.
Rise Culm. Set
Mercury 05:18 11:58 18:38
Venus 04:22 11:16 18:10
Moon 23:14 06:13 13:12
Mars 05:29 12:09 18:48
Jupiter 09:36 15:12 20:48
Saturn 14:16 19:03 23:50

Warning

Never attempt to point a pair of binoculars or a telescope at an object close to the Sun. Doing so may result in immediate and permanent blindness.

Source

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

12 Sep 2017, 05:11 EDT  –  Mercury at greatest elongation west
23 Nov 2017, 21:22 EST  –  Mercury at greatest elongation east
01 Jan 2018, 19:40 EST  –  Mercury at greatest elongation west
15 Mar 2018, 06:18 EDT  –  Mercury at greatest elongation east

Image credit

© NASA/JPL/MESSENGER

Ashburn

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Longitude:
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39.04°N
77.49°W
EST

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