Venus dazzles at dusk and closes in on Neptune

By Ade Ashford

This looping animation depicts a southwesterly view one hour after sunset from 13–31 January 2020 at two-day intervals as seen from the heart of the UK. Dazzling magnitude -4.1 Venus passes just 4 arcminutes (one-fifteenth of a degree) south of Neptune at 8pm GMT on Monday, 27 January 2020, so both planets will fit in the same high-power telescope view from 6pm GMT until they set. Don’t miss magnitude +4.2 naked-eye star Phi (φ) Aquarii that lies within 0.9° east of magnitude +7.9 Neptune throughout the period. Note that the Moon’s apparent size on 27 and 29 January is enlarged for clarity. AN animation by Ade Ashford.
Even casual skywatchers cannot fail to notice brightest planet Venus currently hanging like a lantern above the southwest horizon at nautical dusk, which is presently about 6pm GMT for the centre of the British Isles. On 11 January, Venus crossed the constellation border into Aquarius where it resides for the remainder of the month.

Neptune also currently lies in the constellation of Aquarius. Since the outermost planet shines at magnitude +7.9, it’s too faint to see with the naked eye, but it is a viable binocular target – if you know where to look. Fortunately for skywatchers, Venus makes an increasingly convenient celestial signpost to Neptune’s position as the gap between them closes throughout this month, culminating in a close conjunction on 27 January 2020.

Venus passes one-fifteenth of a degree south of Neptune at 8pm GMT at 27 January 2020, but the pair will be very low in the UK sky. Observers in the British Isles are advised to look at 6pm GMT around the onset of nautical twilight when the two planets are about 19° high in the southwest. Their separation is slightly more than 7 arcminutes at this time. In this simulated one-degree telescope field of view the magnification is 40-50×. AN graphic by Ade Ashford.
While there will no difficulty in identifying Venus in your telescope on the evening of 27 January, Neptune may prove a little more difficult to see in the glare of its planetary sibling. Their difference amounts to a whopping 12 magnitudes, which is another way of saying that Venus is 63,000 times brighter than Neptune! If the outermost planet is lost in Venus’ dazzle, try to spot magnitude +4.2 star Phi (φ) Aquarii in the same field of view shown above.

Venus narrowly misses Phi Aquarii, passing just 56 arcseconds south of the star at 05:20 UT (5:20am GMT) on 28 January. Sadly, this event will not be visible from Western Europe as the timing favours the North Pacific Ocean and the Hawaiian Islands. Neptune has its own close encounter with the star, passing just 2¼ arcminutes north of Phi Aquarii at 8:15pm GMT (20:15 UT) on 10 February 2020; observers in the UK can see the conjunction low in the west-southwest shortly after 6pm GMT that night.

Returning to this month, if you do succeed in viewing Venus and Neptune in the same telescope field of view, do bear in mind that their apparent proximity is merely a line of sight effect. On the evening of 27 January, Venus lies a little more than 167 million kilometres (or 1.117 astronomical units) from Earth, but outermost planet Neptune is a staggering 4,590 million kilometres (or 30.683 astronomical units) distant.