© NASA/JPL/MESSENGER

Mercury reaches highest point in evening sky

Dominic Ford, Editor
From the Inner Planets feed

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The sky at

As seen from Fairfield , Mercury will reach its highest point in the sky in its May–July 2019 evening apparition. It will be shining brightly at mag 0.4.

From Fairfield, this apparition will be well placed but nonetheless tricky to observe, reaching a peak altitude of 18° above the horizon at sunset on 20 Jun 2019.

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 sunset over the course of its the apparition. All times are given in Fairfield local time.

Date Sun
sets at
Mercury
sets at
Altitude
at sunset
Direction
at sunset
27 May 201920:1220:46north-west
30 May 201920:1521:07north-west
02 Jun 201920:1721:2611°north-west
05 Jun 201920:1921:4113°west
08 Jun 201920:2121:5315°west
11 Jun 201920:2322:0217°west
14 Jun 201920:2422:0717°west
17 Jun 201920:2622:1018°west
20 Jun 201920:2722:1018°west
23 Jun 201920:2722:0717°west
26 Jun 201920:2822:0217°west
29 Jun 201920:2821:5515°west
02 Jul 201920:2821:4514°west
05 Jul 201920:2721:3311°west
08 Jul 201920:2621:18west
11 Jul 201920:2521:02west

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

Observing Mercury

The May–July 2019 evening apparition of Mercury
17 Jun 2019 – Mercury at dichotomy
18 Jun 2019 – Mercury reaches highest point in evening sky
23 Jun 2019 – Mercury at greatest elongation east

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 days 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 maximum altitude of between 8° and 21° above the horizon at sunset during each evening apparition, depending on the time of year. During its May–July 2019 apparition, it will peak at 18° above the horizon at sunset on 20 Jun 2019.

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 sunset. 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 May–July 2019 apparition, it will reach a maximum separation of 25° to the Sun's east at greatest elongation.

The optimum time for an apparition of Mercury

The maximum altitude of Mercury during all its evening 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 evening 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 its highest point will be:

Object Right Ascension Declination Constellation Magnitude Angular Size
Mercury 07h57m10s +21°16' Gemini 0.4 8.2"
Sun 06h08m +23°25' Gemini -26.7 31'28"

The coordinates above are given in J2000.0.

The sky on 19 June 2019
Sunrise
05:19
Sunset
20:27
Twilight ends
22:35
Twilight begins
03:10

16-day old moon
Waning Gibbous

96%

16 days old

Planets
Rise Culm. Set
Mercury 07:11 14:40 22:10
Venus 04:23 11:47 19:12
Moon 22:35 02:29 07:09
Mars 07:08 14:38 22:07
Jupiter 19:29 00:13 04:52
Saturn 21:38 02:23 07:05
All times shown in EDT.

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

19 Jun 2019  –  Mercury reaches highest point in evening sky
23 Jun 2019  –  Mercury at greatest elongation east
09 Aug 2019  –  Mercury at greatest elongation west
12 Aug 2019  –  Mercury reaches highest point in morning sky

Image credit

© NASA/JPL/MESSENGER

Fairfield

Latitude:
Longitude:
Timezone:

41.14°N
73.26°W
EDT

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