© NASA/JPL/MESSENGER

Mercury at greatest elongation west

Wed, 17 May 2017 at18:58 EDT(99 days ago)
22:58 UTC

Dominic Ford, Editor
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The sky at

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

From Ashburn (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'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 west.

Mercury in coming weeks

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

20 Apr 2017 01:48 EDT – Mercury at inferior solar conjunction
17 May 2017 18:58 EDT – Mercury at greatest elongation west
23 May 2017 10:35 EDT – Mercury at dichotomy

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
10 May 201706:0005:09north-west
17 May 201705:5404:56north-west
24 May 201705:4804:4910°north-west
31 May 201705:4504:4710°north-west
07 Jun 201705:4304:53north-west
14 Jun 201705:4205:10north-west
21 Jun 201705:4305:41north-west
28 Jun 201705:4506:20-6°north-west
05 Jul 201705:4807:01-11°west
12 Jul 201705:5207:36-17°west
19 Jul 201705:5808:04-21°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 02h01m40s +08°48' Pisces 0.4 8.2"
Sun 03h38m +19°28' Taurus -26.7 31'37"

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 May 17, the ecliptic is inclined at 34° 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 Ashburn.

The sky on 17 May 2017
Sunrise 05:54
Sunset 20:16
Twilight ends
22:05
Twilight begins
04:05

21-day old moon
Age of Moon
21 days

All times shown in EDT.
Rise Culm. Set
Mercury 04:56 11:28 17:59
Venus 03:58 10:13 16:29
Moon 01:01 06:08 11:15
Mars 07:07 14:33 22:00
Jupiter 16:29 22:18 04:12
Saturn 22:22 03:13 07:59

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

17 May 2017, 18:58 EDTMercury at greatest elongation west
23 May 2017, 10:35 EDTMercury at dichotomy
21 Jun 2017, 06:36 EDTMercury at greatest brightness
21 Jun 2017, 10:02 EDTMercury at superior solar conjunction

Image credit

© NASA/JPL/MESSENGER

Ashburn

Latitude:
Longitude:
Timezone:

39.04°N
77.49°W
EDT

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