On Wednesday 18 January, brightest asteroid 4 Vesta comes to opposition in the constellation of Cancer bordering on Gemini, ideally placed for observation by Northern Hemisphere skywatchers. Two days later, Vesta makes its closest approach to Earth for this year at a distance of 1.5217 astronomical units, or 141 million miles (227 million kilometres).
With a mean diameter of 326 miles (525 kilometres) and an orbital period of 3.63 years, Vesta is the second-largest main-belt asteroid after dwarf planet 1 Ceres that lie between the orbits of Mars and Jupiter. This year marks the 210th anniversary of Vesta’s discovery by German astronomer Heinrich Wilhelm Olbers.
Presently shining at close to magnitude +6.3, Vesta is therefore on the verge of naked-eye visibility for eagle-eyed observers under dark, moonless skies. For the rest of us with average vision, the asteroid is still an easy binocular object — if you know exactly where to look.Prominent stars Castor and Pollux in eastern Gemini may be found due south at midnight in mid-January (or 11pm GMT by month’s end), two-thirds of the way from the southern horizon to overhead as seen from the UK. The celestial twins lie just 4½ degrees apart, so the pair can be seen in the same field of view of a typical 10×50 binocular.
If an imaginary line drawn from Castor through Pollux is extended a similar distance, it marks the position of Vesta around 24 January — an additional aid to identifying the asteroid around that time. Given the brightness of Vesta, it will be easy to spot the interloper amid the stars of Gemini shown in the finder chart above that shows binocular stars down to magnitude +8.
As a further guide to identifying Vesta, the asteroid passes within half a degree (less than a lunar diameter) north of magnitude +3.6 naked-eye star kappa (κ) Geminorum early on 3 February. Pollux and κ Geminorum are separated by just 3.6 degrees, another binocular pairing that will help you find Vesta in early February.
Inside the magazine
For a comprehensive guide to observing all that is happening in this month’s sky, tailored to Western Europe, North America and Australasia, obtain a copy of the January 2017 edition of Astronomy Now.
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