While antipodean observers are enjoying views of the totally eclipsed Blue Moon in Cancer (The Crab) on their night of 31 January/1 February, observers should not forget to look out for 946-km-wide dwarf planet 1 Ceres at opposition less than a span of an outstretched hand at arm’s length north of the lunar disc in the same constellation.
On Thursday, 1 February at 15:10 UT (3:10pm GMT), 1 Ceres, the nearest dwarf planet and the largest minor planet inside the orbit of Neptune, passes closest to Earth for this year. At this instant, the minor planet is 1.602 astronomical units, or 239.6 million kilometres (148.9 million miles) from our world. This is the closest approach of 1 Ceres to Earth since the evening of 25 February 2009 when the body was 1.583 astronomical units, or 236.8 million kilometres (147.2 million miles) distant.
Ceres reaches opposition in the small hours of 31 January and shines at its peak magnitude of +6.9 for 2018 (fading to +7.3 by the beginning of March), hence it’s a comfortable binocular or small telescope target — if you know exactly where to look. Viewed from the UK, the dwarf planet is currently highest in the sky close to 1am GMT, some 65 degrees high in the south as seen from the centre of the British Isles.
When and how to see Ceres
Since first quarter Moon occurred on the evening of Wednesday, 24 January, the light from the waxing gibbous lunar disc makes the task of finding 1 Ceres slightly more difficult, but the minor planet is still within the grasp of a typical 10×50 binocular. To identify it, one ideally needs not only a moonless night when the dwarf planet is high above the horizon murk, but a location that is largely free from light pollution too.
To ensure a successful identification, find a safe area as far removed from streetlights as you can and allow your eyes at least 15 minutes to become fully adapted to the dark. If the Moon is up, try to hide its glare behind a fence or wall. Assuming that you have printed both the chart above and the detailed PDF version, study them under a dim red light so as to preserve your night vision. Try to memorise the positions of faint stars in the vicinity of 1 Ceres and, if you’re fortunate to have a run of clear nights, you’ll be able to follow its retrograde motion through Cancer. Bear in mind that the dwarf planet is indistinguishable from a star, so only its shifting position from night to night will betray it.The position of 1 Ceres on the chart is for 0h UT (12am GMT) on the dates shown, so make an estimate of its position on intervening dates and times. Telescope users may also use the daily equatorial coordinates tabulated below, suitably interpolated for intermediate times. Astrophotographers may be interested to know that the dwarf planet lies close to magnitude +12.2 galaxy NGC 2789 (α=09h 15.0m, +29° 44′ J2000.0) in the small hours of 28 January. Ceres lies in the same 10×50 binocular field as magnitude +4 star iota (ι) Cancri (α=08h 46.7m, +28° 46′ J2000.0) once moonless UK skies return around 8 February, passing 3⅓ degrees north of the star on 3 March.