This Week’s Sky at a Glance, 2020 June 20 – 27 ~by Curt Nason
Seasons are the result of the earth’s rotational axis being tilted about 23.5 degrees off the vertical, with respect to its orbit. The first day of astronomical summer is this Saturday. The “astronomical” qualification is used because meteorologists have taken to confusing people with meteorological seasons based on temperatures. Meteorological summer in the northern hemisphere includes June, July and August because they have the highest average temperatures for the year.
On the summer solstice, the Sun rises and sets at its most northerly points on the horizon. For those of us at 45 degrees latitude, at midday (1:20 pm in Moncton) the Sun is about 67.5 degrees above the southern horizon; its highest altitude for the year. If we lived at latitude 23.5 degrees the Sun would be directly overhead at midday on the solstice. Several millennia ago the Sun was “in” the constellation Cancer on the solstice, hence that latitude is marked on maps as the Tropic of Cancer. The dim constellation does resemble a crab somewhat, but there is speculation that the Sun’s forth and back movement along the horizon at that time of year was reminiscent of a crab’s sideways walk.
Prior to being in Cancer at the start of summer, the Sun was in Leo. Lions tended to gather by the Nile in the dry season around the solstice. Now the summer solstice point on the ecliptic, the Sun’s path through the constellations, lies in Taurus, just within its boundary with Gemini. The roaming solstice is due to Earth’s axis wobbling like a top, making one revolution every 25,800 years in what we call the precession of the equinox. Enjoy your summer, whenever it starts.
This Week in the Solar System
Saturday’s sunrise in Moncton is at 5:27 am and sunset will occur at 9:13 pm, giving 15 hours, 46 minutes of daylight (5:36 am and 9:15 pm in Saint John). Next Saturday the Sun will rise at 5:30 am and set at 9:14 pm, giving 15 hours, 44 minutes of daylight (5:38 am and 9:16 pm in Saint John). Astronomical summer begins at 6:44 pm this Saturday.
The Moon is new early Sunday morning and, with ideal sky conditions and binoculars, the 18-hour crescent might be spotted in early twilight that evening. Mercury will be a binocular width to its left but likely too dim to be seen in bright twilight. By midweek Jupiter and Saturn are rising less than 90 minutes after sunset, but they will not give good views in a telescope until well past midnight. Mars will give its best views in early morning twilight, and while you are there look for the crescent Venus rising just after 4 am.
With astronomy meetings and outreach activities on hold, you can watch the local Sunday Night Astronomy Show at 8 pm and view archived shows.
Questions? Contact Curt Nason.