Dominic Ford, Editor
From the Inner Planets feed
Venus will be well placed for observation in the evening sky, shining brightly at mag -4.3.
From Ashburn, it will be difficult to observe as it will appear no higher than 16° above the horizon. It will become visible at around 20:18 (EST) as the dusk sky fades, 16° above your western horizon. It will then sink towards the horizon, setting 1 hour and 46 minutes after the Sun at 21:45.
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.
These apparitions take 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.
On this occasion, it lies 45° to the Sun's east.
Venus in coming weeks
The key moments in this apparition of Venus are as follows:
|15 Aug 2018 01:09 EDT||– Venus at dichotomy|
|17 Aug 2018 03:58 EDT||– Venus at greatest elongation east|
|25 Sep 2018 00:17 EDT||– Venus at greatest brightness|
|26 Oct 2018 10:11 EDT||– Venus at inferior solar conjunction|
After greatest elongation, 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 Venus will remain up after sunset each night; all times are given in Ashburn local time.
|Altitude of Venus
|Direction of Venus
|10 Aug 2018||20:08||22:00||20°||west|
|17 Aug 2018||19:59||21:46||19°||south-west|
|24 Aug 2018||19:49||21:30||18°||south-west|
|31 Aug 2018||19:39||21:14||16°||south-west|
|07 Sep 2018||19:28||20:56||14°||south-west|
|14 Sep 2018||19:17||20:37||13°||south-west|
|21 Sep 2018||19:05||20:15||11°||south-west|
|28 Sep 2018||18:54||19:50||8°||south-west|
|05 Oct 2018||18:43||19:22||6°||south-west|
|12 Oct 2018||18:32||18:51||2°||south-west|
|19 Oct 2018||18:22||18:17||-1°||south-west|
A graph of the angular separation of Venus from the Sun around the time of greatest elongation is available here.
The position of Venus when it reaches greatest elongation will be:
|Object||Right Ascension||Declination||Constellation||Magnitude||Angular Size|
The coordinates above are given in J2000.0.
At each apparition, Venus reaches a similar separation from the Sun – around 48°. This distance 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 than others.
It appears most favourably in the evening sky around the time of the local spring equinox, and most favourably in the morning sky around the local autumn equinox.
These dates are reversed in the northern and southern hemispheres, such that a good apparition in one hemisphere will not be easily observable from the other.
This is comes about because Venus always lies close to the line of the ecliptic, shown in yellow in the planetarium above. 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.
When Venus remains in the sky for a few hours after the Sun has set, its altitude above the horizon depends on two factors.
One is its angular separation from the Sun. But equally important is how steeply the line of the ecliptic is inclined to the horizon.
If Venus is widely separated from the Sun along the ecliptic, this may not translate into a high altitude if the ecliptic meets the horizon at a very shallow angle, running almost parallel to it.
Conversely, if the ecliptic is almost perpendicular to the horizon, a much smaller separation from the Sun may place Venus higher in the sky.
The inclination of the ecliptic plane to the horizon at Ashburn varies between 74° (sunset at the spring equinox) and 27° (sunset at the autumn equinox). On August 17, the ecliptic is inclined at 30° 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 Ashburn.
|The sky on 17 August 2018|
6 days old
All times shown in EDT.
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.
|17 Aug 2018, 03:58 EDT||– Venus at greatest elongation east|
|06 Jan 2019, 01:02 EST||– Venus at greatest elongation west|
|24 Mar 2020, 03:31 EDT||– Venus at greatest elongation east|
|13 Aug 2020, 09:03 EDT||– Venus at greatest elongation west|
© NASA/Ricardo Nunes