Dates: June 4-23
Mercury emerges out of the evening twilight at waning gibbous phase in early
June and reaches maximum elongation (24°) on June 21. At northern
observatories, Mercury will reach up to 7° above the horizon a minimum
nautical twilight. Sodium and potassium illumination will be ideal
through June 23, but will fade quickly after that.
The bright standard star Regulus (alpha Leo) will set approximately 2 hours
after Mercury at nearly the same azimuth, making it ideal for measuring
atmospheric extinction once Mercury has set.