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BAA Observing calendar

Venus at greatest elongation east

Friday, 2018, August 17 - 08:58

Dominic Ford – originally published on In-The-Sky.org

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Across much of the world Venus will be well placed for observation in the evening sky, shining brightly at mag -5.1.

From London (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 6° above the horizon at dusk.

Venus in coming weeks

Over coming weeks, the distance between Venus and the Sun will decrease each night, and it will gradually sink back into the Sun's glare, as the table below indicates (all times given in London local time).

Date Sun
sets at
Venus
sets at
Altitude of Venus
at sunset
Direction of Venus
at sunset
10 Aug 2018 20:29 21:48 11° west
17 Aug 2018 20:16 21:28 10° west
24 Aug 2018 20:02 21:06 south-west
31 Aug 2018 19:46 20:44 south-west
07 Sep 2018 19:31 20:21 south-west
14 Sep 2018 19:15 19:56 south-west
21 Sep 2018 18:59 19:29 south-west
28 Sep 2018 18:42 19:00 south-west
05 Oct 2018 18:27 18:30 south-west
12 Oct 2018 18:11 17:59 -2° south-west
19 Oct 2018 17:56 17:28 -4° south-west

Venus's position

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

Object Right Ascension Declination Constellation Magnitude Angular Size
Venus 12h34m30s -05°21' Virgo -5.1 24.3"
Sun 09h45m +13°28' Leo -26.7 31'35"

The coordinates above are given in J2000.0.

The geometry of Venus's orbit

Venus never ventures far from the Sun in the sky since its orbit lies closer to the Sun than the Earth's. To see it, we are always looking inwards towards the center of the solar system, close to the Sun. Venus is said to be at greatest enlongation when it passes either of the two points along its orbit where it appears at greatest separation from the Sun.

As it orbits the Sun, it appears alternately in the morning and evening skies. When it is to the east of the Sun, it rises and sets a short time after the Sun, becoming visible in early evening twilight. When it is to the west of the Sun, it rises and sets a short time before the Sun, becoming visible shortly before sunrise.

Venus is so bright and conspicuous – the third brightest object in the sky after the Sun and Moon – that it is often called the morning or evening star.

Seasonal variation

Each time it appears in the morning or evening sky, Venus reaches roughly the same angular separation from the Sun – this time peaking at a distance of 45° at greatest elongation. This angle is set by the geometry of how big Venus's orbit is, and how far away it is from the Earth. Nonetheless, some times of the year are more favourable for viewing Venus from London than others.

It appears most favourably in the evening sky around the spring equinox, and most favourably in the morning sky around the autumn equinox.

This is because it always lies close to a line across the sky called the ecliptic. 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.

The altitude at which Venus appears above the horizon at sunrise or sunset depends how steeply the line of the ecliptic is inclined to the horizon. If the plane of the ecliptic meet the horizon at a very shallow angle, Venus will rise or set along a line which is almost parallel to the horizon, and a separation of 45° from the Sun along this line would correspond to a very low altitude in the sky.

The inclination of the ecliptic plane to the horizon at London varies between 61° (sunset at the spring equinox) and 15° (sunset at the autumn equinox). On August 17, the ecliptic is inclined at 17° to the western sunset horizon, as shown by the yellow line in the planetarium view above, meaning that on this occasion Venus is poorly placed for viewing from London.


The details of this observing event were provided courtesy of In-The-Sky.org

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