Venus at dichotomy

2019 Jan 5

Venus will be well placed for observation in the dawn sky, shining brightly at mag -4.4.

From London, it will rise at 04:10 (BST) – 3 hours and 57 minutes before the Sun – and reach an altitude of 20° above the southern horizon before fading from view as dawn breaks at around 07:40.

Venus’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 weeks each time it reaches greatest separation from the Sun – moments referred to as greatest elongation.

On these occasions, however, 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 or evening star.

Venus’s phase

Venus’s phase varies depending on its position relative to the Earth. When it passes between the Earth and Sun, for example, the side that is turned towards the Earth is entirely unilluminated, like a new moon.

Conversely, when it lies opposite to the Earth in its orbit, passing almost behind the Sun, it appears fully illuminated, like a full moon. However, at this time it is also at its most distant from the Earth, so it is actually fainter than at other times.

Venus shows an intermediate half phase – called dichotomy – at roughly the same moment that it appears furthest from the Sun, at greatest elongation. The exact times of the two events may differ by a few hours, only because Venus’s orbit is not quite perfectly aligned with the ecliptic.

Venus in coming weeks

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

26 Oct 2018 15:11 BST  – Venus at inferior solar conjunction
30 Nov 2018 02:23 GMT  – Venus at greatest brightness
05 Jan 2019 19:16 GMT  – Venus at dichotomy
06 Jan 2019 06:02 GMT  – Venus at greatest elongation west

Over coming weeks, the distance between Venus and the Sun will decrease each night as it sinks back into the Sun’s glare. The table below lists how long before sunrise Venus will rise each night; all times are given in London local time.

Date Sun
sets at
Venus
rises at
Altitude of Venus
at sunrise
Direction of Venus
at sunrise
29 Dec 2018 08:05 03:58 23° west
05 Jan 2019 08:04 04:08 21° west
12 Jan 2019 08:01 04:19 19° west
19 Jan 2019 07:55 04:31 18° west
26 Jan 2019 07:47 04:42 16° west
02 Feb 2019 07:37 04:53 15° west
09 Feb 2019 07:25 05:01 13° west
16 Feb 2019 07:12 05:07 12° west
23 Feb 2019 06:58 05:11 11° west
02 Mar 2019 06:44 05:11 10° west
09 Mar 2019 06:28 05:09 west

A graph of the phase of Venus is available here.

Venus’s position

The coordinates of Venus when it reaches dichotomy will be:

Object Right Ascension Declination Constellation Angular Size
Venus 15h46m20s -16°21' Libra 24.9"
Sun 19h04m -22°36' Sagittarius 32'31"

The coordinates above are given in J2000.0.

This entry in the observing calendar was provided by In-The-Sky.org

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