This is the view low to the southwest at 6 pm GMT on 19th January as seen from the UK, the night that Mars is closest to fainter and much more distant planetary sibling Neptune. To the naked eye, magnitude +1.4 Mars will lie about the span of an outstretched hand at arm’s length above the horizon, the brightest object in the area. AN graphic by Ade AshfordNovice observers wishing to see outermost planet Neptune with small telescopes often have difficulty locating it, not because it’s excessively faint — slightly brighter than magnitude +8, so 10×50 binoculars will reveal it under dark, moonless skies — but due to its star-like appearance. Higher magnifications will reveal its pale blue 2.2-arcsecond disc, distinguishing it from background stars of similar brightness. However, a very convenient celestial marker in the form of a close conjunction with planetary sibling Mars makes locating Neptune very easy in a telescope on Monday, 19th January.
This one-degree-wide view centred on Mars at 6 pm on 19th January shows the field stars around Neptune that you will see in a low-power telescope field as an aid to identification. While Mars will only be in this position for the night of the 19th, Neptune will lie 0.6° — slightly more than the diameter of the Full Moon — north of magnitude +4.8 naked-eye star sigma (σ) Aquarii for several nights. AN graphic by Ade AshfordSet against the constellation backdrop of Aquarius low to the southwest at the time of their closest approach — which is, of course, a line of sight effect — Mars will be 2.06 astronomical units (191 million miles; 307 million km.) distant and displaying a tiny 4.6-arcsecond gibbous disc, while Neptune is a colossal 30.75 AU (2859 million miles) away!
Observers in the British Isles should have their binoculars or telescopes setup facing an unobscured southwest horizon by 5:30 pm GMT as twilight fades enough for the stars to be visible. As you observe this planetary juxtaposition with an angular separation of just ¼° in the eyepiece, contemplate the contrast in physical makeup of these two worlds — Mars is rocky and more Earth-like though half the size of our planet, while Neptune is gaseous and four times the size of Earth.
For an even greater image contrast, brilliant Venus also passes just ¾° away from Neptune on the evening of February 1st. More on that next month.
For three evenings from 26–28 February 2020, observers in Western Europe including the British Isles can watch the waxing crescent Moon’s changing configuration with brightest planet Venus in the west-southwest at dusk. The pair are closest for UK-based observers on the evening of Thursday, 27 February, simultaneously visible in low-power binoculars.
Inner planets Mercury and Venus are currently approaching conjunction, low to the southwest horizon shortly after sunset. If you have an extra clear sky, then Mars completes the scene.
