This Week’s Sky at a Glance, January 14 – 21 ~ by Curt Nason
Earlier this week I noticed a halo around the Moon on two nights. A clear evening sky in winter is always spectacular because several of the brightest stars are near Orion, and the early sunsets give us plenty of time to enjoy them if we don’t mind the cold. With a near full Moon in the area and the right weather conditions, the addition of a halo creates the risk of a frozen tongue as we stare open-mouthed at the scene. On Tuesday I noticed the halo was cutting Orion in half, with the belt just outside the circle.
Halos are caused by sunlight or bright moonlight shining through hexagonal ice crystals in the atmosphere. The light refracts, or bends, through two sides and leaves the crystal at an angle of 22 degrees from the direction it arrived. If we are looking at the Moon, we see that light at an angular distance of 22 degrees. This is about equal to a hand span at arm’s length, from thumb tip to the tip of your pinkie, barring any carpentry accidents. The ice crystals can form any time of year – it is cold up there – but conditions are most prevalent through winter. They are often colourful during daylight with the bright sunshine, but we need the near full Moon to reflect enough sunlight for a halo to be visible and then it is usually not bright enough to see colours. The full Moon is about 11 times brighter than it is at first quarter.
Seven of the 15 brightest stars are in or around Orion, forming the Winter Circle or Hexagon. If you like geometry, three of those also form the equilateral Winter Triangle. Those are orange Betelgeuse in Orion’s shoulder, plus Sirius and Procyon, the two Dog Stars. Betelgeuse is near the centre of the Winter Circle, surrounded by Sirius, Procyon, Pollux in Gemini, Capella in Auriga, Aldebaran in Taurus, and Rigel in Orion.
This Week in the Solar System
Saturday’s sunrise in Moncton is at 7:58 am and sunset will occur at 4:59 pm, giving 9 hours, 1 minute of daylight (8:00 am and 5:07 pm in Saint John). Next Saturday the Sun will rise at 7:53 am and set at 5:09 pm, giving 9 hours, 16 minutes of daylight (7:55 am and 5:16 pm in Saint John).
The Moon is at third quarter on Thursday, rising just after midnight and setting a little before noon. Also on Thursday, Mercury reaches its greatest elongation from the Sun in the morning sky. Since you are up checking that out on Thursday, have a look at Jupiter with binoculars or a scope to see all four of its Galilean moons lined up on one side. Then try Saturn to the upper right of Mercury. Its rings are at their best viewing in more than a decade. A small scope will now show Venus in its half-lit phase, especially in evening twilight when the bright planet is less glaring. Orange Mars is less than 10 degrees east of Venus.
RASC NB, the provincial astronomy club meets on January 21 at 1 pm in the Rockwood Park Interpretation Centre. All are welcome and free to attend.
Questions? You can contact Curt Nason here.