Venus at greatest elongation west

2015 Oct 26

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

Venus will be well placed for observation in the dawn sky, shining brightly at mag -5.1.

From London (click to change), it will be visible in the dawn sky. It will rise at 02:19 (GMT), 4 hours and 23 minutes before the Sun, and attain an altitude of 34° above the south-eastern horizon before fading from view as dawn breaks at around 06:19.

Venus in coming weeks

Over coming weeks, the distance between Venus and the Sun will decrease each morning, 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
rises at
Venus
rises at
Altitude of Venus
at sunrise
Direction of Venus
at sunrise
19 Oct 2015 07:28 03:10 36° south-east
26 Oct 2015 06:41 02:19 36° south-east
02 Nov 2015 06:53 02:30 35° south-east
09 Nov 2015 07:06 02:43 34° south-east
16 Nov 2015 07:18 02:58 33° south-east
23 Nov 2015 07:30 03:13 31° south-east
30 Nov 2015 07:41 03:31 28° south
07 Dec 2015 07:50 03:49 26° south
14 Dec 2015 07:57 04:08 24° south
21 Dec 2015 08:03 04:27 21° south
28 Dec 2015 08:05 04:46 19° south

Venus’s position

The exact position of Venus when it reaches greatest elongation is as follows:

Object Right Ascension Declination Constellation Magnitude Angular Size
Venus 11h08m00s +05°33′ Leo -5.1 24.1″
Sun 14h00m -12°16′ Virgo -26.7 32’10”

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 46° 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 46° 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° (sunrise at the autumn equinox) and 15° (sunrise at the spring equinox). On October 26, the ecliptic is inclined at 56° to the eastern dawn horizon, as shown by the yellow line in the planetarium view above, meaning that on this occasion Venus is very favourably placed for viewing from London.


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

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