In an earlier post, I wrote about Mercury’s prominent evening show as seen from the Southern Hemiphere at the end of July. The innermost planet was also in conjunction with Regulus, the brightest star in the constellation of Leo, late that month.
Having passed inferior conjunction, lost from view in the glare between the Earth and Sun on 26 August, Mercury returns to the morning sky in an apparition favouring Northern Hemisphere observers. Interestingly, the little planet flirts with Regulus yet again this month, passing just 0.6 degrees (little more than the width of a full Moon) south of the star on 10 September.
Mercury lies in Leo for much of the month, moving into Virgo on 26 September. The innermost planet emerged into the dawn sky an hour before sunrise around 5 September for observers in the UK — the first of two good morning apparitions for Northern Hemisphere observers this year. Mercury attains a greatest westerly elongation of 18 degrees from the Sun on 12 September when it can be found 6 degrees high above the east-northeast horizon an hour before sunrise as seen from the centre of the British Isles.
Mars is a magnitude +1.8 object in the constellation of Leo throughout the month. The Red Planet rises 100 minutes before the Sun in mid-September as seen from the heart of the UK, but is only visible in a dark sky from the end of the month. Magnitude -0.8 Mercury lies just 21 arcminutes (one-third of a degree) from Mars as seen from the British Isles before sunrise on 17 September.
Brightest planet Venus starts the month in the constellation of Cancer, passing into Leo on 10 September. The planet shines at magnitude -3.9 throughout the month, its elongation from the Sun decreasing from 32 to 25 degrees over the course of September.
An hour before UK sunrise on 18 September finds the 27-day-old waning crescent Moon 3 degrees below Venus while first-magnitude star Regulus lies almost a degree above the thin lunar crescent. Venus is actually occulted by the Moon around 01h UT on 18 September as seen from Indonesia, Australia and New Zealand.