Third-magnitude star Zeta (ζ) Tauri is occulted (hidden) by the 19-day-old waning gibbous Moon in the small hours of Saturday, 19 October 2019 as seen from the whole of the British Isles. This illustration shows the changing aspect of the star’s occultation as seen from London and Edinburgh, clearly demonstrating the effect that geographical latitude has on the timing of the event. Note that all times are in British Summer Time (Universal Time + 1 hour). AN graphic by Ade Ashford.When a nearby astronomical body passes between the observer and a more distant object, see say that an occultation (from the Latin occulo, ‘to hide’) is taking place. Since the Moon is our nearest celestial neighbour, it regularly passes in front of planets and stars.
Although none of the four first-magnitude zodiacal stars – Aldebaran in Taurus, Regulus in Leo, Spica in Virgo and Antares in Scorpius – are hidden by the Moon this year, planets Venus, Mars, Jupiter and Saturn are all subject to lunar occultation in 2019. (Jupiter is next for the British Isles, occulted by the Moon in daylight on 28 November.)
Fainter stars are clearly more plentiful and the Moon has already occulted many of these so far this year. Fortunately for observers in the British Isles and Northwestern Europe, the brightest naked-eye star subject to a lunar occultation this month is third-magnitude Zeta (ζ) Tauri in the small UK hours of Saturday, 19 October. See the illustration above for timings of the star’s disappearance and reappearance as seen from London and Edinburgh.
As with all occultation observations, make sure that you are ready at the eyepiece of your telescope a few minutes before the appointed times to ensure success, particularly if you live some distance from the cities mentioned. The Moon’s west-to-east motion against the background stars – at a rate equivalent to its own diameter every hour – is particularly evident when an occultation is about to occur and quite spectacular when a conspicuous star or planet disappears/reappears at the Moon’s dark limb.