BBC Sky at Night Magazine
Mercury near Spica during the day
BEST TIME TO SEE: 22 September at 14:30 BST (13:30 UT)
On 22 September, mag. 0.0 Mercury lies due south at 14:30 BST (13:30 UT), attaining an altitude of 26˚ from the centre of the UK. At this time, it’s 0.3˚ northeast of mag. +1.0 Spica (Alpha (a) Virginis). Despite being daylight, given clear skies it may still be possible to see both planet and star. The easiest way to locate them in daylight is to offset from another astronomical object, in this case the Sun. Precautions must be used in order to do this.
A telescope on an aligned equatorial mount with setting circles can be used. The telescope must be filtered using a full aperture certified solar filter. Finders must be capped or filtered too. Centre on the Sun and set the setting circles to the Sun’s co-ordinates, then offset to Mercury. Ensure the scope isn’t pointing at the Sun before removing the filters or caps. As a double-check, the scope should be pointing east (left) of the Sun.
At 14:30 BST on 22 September the Sun’s co-ordinates are
RA 11h 59m 00s, dec. +00˚ 06 minutes 36 seconds. The RA of Mercury will be 13h 26m 18s, dec. –10˚ 59 minutes 42 seconds. Apps or planetarium programs such as Stellarium can be used to give the positions at other times of the day.