As Venus moves along on its orbit around the Sun, from Earth it is disappearing more and more each evening into the Sun’s glare. At the same time, mighty Mercury is fast pulling away and will reach greatest Eastern elongation of 23.6 degrees from the Sun on Thursday 4th of June. Around this time then, is the best time to view Mercury since it will be at its highest point above the horizon at sunset in the evening sky. Warning! — NEVER use a telescope or binoculars while the Sun is still above the horizon, instant blindness can occur.
The closest planet to the Sun, Mercury’s orbital distance is about 58 million kilometres, about one third of the Earth’s distance. This means it’s ‘Year’ (length of time it takes to orbit the Sun) is just 88 days!
Visibility problems increase at higher latitudes like ours, where Mercury’s proximity to an often cloud-strewn horizon makes naked eye viewing a real challenge. However, due to the noticeably more transparent atmosphere due to the reduction in air travel and industry during the Covid-19 pandemic lockdown, it’s a little easier to spot on the horizon.
The spring (evening) and autumn (morning) appearances of Mercury are the best times to see it, when the geometry of the Earth, Sun and Mercury ensures the planet is at its highest above the horizon, improving chances of spotting it. Knowing the times and dates of its appearance is essential to have a chance of seeing this elusive planet. This information is found in most astronomical journals and web sites. (You may also contact the club for help). As for Thursday 4th, by 22:25 the sky will have ‘darkened’ sufficiently enough that you should be able to spot it. At this time, it will be 10 degrees in Altitude (about the same height as a closed fist held at arm’s length on the horizon) and +299deg Azimuth (WNW) – use a compass to find this bearing. Alternatively, there are several astronomy Apps for your phone that are great for locating celestial objects in real time as you hold it up to the sky. Sky View, Sky Walk, Sky Safari are but some available. My personal favourite is Stellarium. While this does not have a ‘live view’ feature, it is a fantastic planetarium program for PC and phone that I use regularly when planning my observing.
Limerick Astronomy Club
email limerickastronomyclub @gmail.com