© NASA/Ricardo Nunes

Venus at greatest elongation west

Dominic Ford, Editor
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Venus will reach its greatest separation from the Sun in its 2015–2016 morning apparition. It will be shining brightly at mag -4.4.

From Ashburn , this apparition will be well placed and prominent, reaching a peak altitude of 42° above the horizon at sunrise on 21 Oct 2015.

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 Venus will appear at sunrise over the course of the apparition. All times are given in Ashburn local time.

Date Sun
sets at
Venus
rises at
Altitude
at sunrise
Direction
at sunrise
22 Aug 201506:3105:52west
01 Sep 201506:4004:5420°north-west
11 Sep 201506:4904:1229°north-west
21 Sep 201506:5803:4735°north-west
01 Oct 201507:0703:3439°north-west
11 Oct 201507:1703:3142°north-west
21 Oct 201507:2703:3542°north-west
31 Oct 201507:3803:4441°north-west
10 Nov 201506:4902:5640°west
20 Nov 201507:0003:1138°west
30 Nov 201507:1003:2835°west
10 Dec 201507:1903:4632°west
20 Dec 201507:2604:0629°west
30 Dec 201507:3004:2726°west
09 Jan 201607:3004:4723°west

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

Observing Venus

The 2015–2016 morning apparition of Venus
15 Aug 2015 – Venus at inferior solar conjunction
20 Sep 2015 – Venus at greatest brightness
21 Oct 2015 – Venus reaches highest point in morning sky
25 Oct 2015 – Venus at dichotomy
26 Oct 2015 – Venus at greatest elongation west

Venus'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 a few months each time it reaches greatest separation from the Sun – moments referred to as greatest elongation.

On these occasions, Venus is so bright and conspicuous that it becomes the third brightest object in the sky after the Sun and Moon. It is often called the morning star or the evening star.

These apparitions repeat roughly once every 1.6 years, taking place alternately in the morning and evening skies, depending whether Venus 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.

At each apparition, Venus reaches a maximum separation from the Sun of around 48°. However, some times of the year are more favourable for viewing Venus than others. From Ashburn, it reaches a peak altitude of between 23° and 45° above the horizon at sunrise during each morning apparition, depending on the time of year. During its 2015–2016 apparition, it will peak at 42° above the horizon at sunrise on 21 Oct 2015.

This variability over the course of the year is due to the inclination of the ecliptic to the horizon.

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, Venus 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 Venus 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 Venus 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 Venus 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, Venus 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 Venus in one hemisphere will usually be badly placed in the other.

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

The optimum time for an apparition of Venus

The maximum altitude of Venus 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.

For this reason, the day when Venus reaches its widest separation from the Sun (greatest elongation) is not necessarily the same day when it appears highest in the sky at sunrise. Venus typically appears highest in the sky a few days or weeks closer to the autumn equinox than the moment of greatest elongation.

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 October 26, the ecliptic is inclined at 69° to the eastern dawn horizon, as shown by the yellow line in the planetarium above, meaning that this apparition of Venus will be well placed and prominent, reaching a peak altitude of 42° above the horizon at sunrise.

Venus's position

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

Object Right Ascension Declination Constellation Magnitude Angular Size
Venus 11h08m00s +05°33' Leo -4.4 24.1"
Sun 14h00m -12°16' Virgo -26.7 32'10"

The coordinates above are given in J2000.0.

The sky on 26 October 2015
Sunrise
07:29
Sunset
18:16
Twilight ends
19:46
Twilight begins
06:00

13-day old moon
Waxing Gibbous

97%

13 days old

Planets
Rise Culm. Set
Mercury 06:18 12:03 17:48
Venus 03:39 10:00 16:21
Moon 18:08 00:39 05:58
Mars 03:53 10:14 16:35
Jupiter 03:35 09:59 16:24
Saturn 09:59 14:57 19:54
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

26 Oct 2015  –  Venus at greatest elongation west
12 Jan 2017  –  Venus at greatest elongation east
05 Feb 2017  –  Venus reaches highest point in evening sky
03 Jun 2017  –  Venus at greatest elongation west

Image credit

© NASA/Ricardo Nunes

Ashburn

Latitude:
Longitude:
Timezone:

39.04°N
77.49°W
EDT

Color scheme