This Week’s Sky at a Glance, September 15 – 22 ~by Curt Nason
This is the time of year when the evening sky seems static; the stars are in the same place night after night when they appear in twilight. As you can see below, the Sun sets about two minutes earlier each evening. With reference to the stars, Earth rotates once every 23 hours, 56 minutes and 4 seconds. But since our clocks are based on a 24-hour solar day rather than the sidereal day, the stars rise about 4 minutes earlier each evening. The rate of earlier sunsets this time of year cancels much of that. Although the stars rise earlier we see also see them sooner. That is a bonus because many of the finest objects to observe in a telescope are prominent now, particularly the Milky Way.
The opposite occurs in spring when the later sunsets add to the earlier rising of stars. The constellations seem to fly past over a month or two, much to the chagrin of those who delight in observing the distant galaxies that abound in those constellations. Earth’s motion around the Sun results in many of the constellations being seasonal. For example, we currently see Orion in the southeast before sunrise. Come January it will be there after sunset and stick around in the evening sky until mid-spring. Those constellations near the north are circumpolar, meaning they never set and we see them year round. There are 22 constellations in the southern hemisphere sky that we see no part of at all from New Brunswick.
This Week in the Solar System
Saturday’s sunrise in Moncton is at 6:57 am and sunset will occur at 7:30 pm, giving 12 hours, 33 minutes of daylight (7:02 am and 7:35 pm in Saint John). Next Saturday the Sun will rise at 7:06 am and set at 7:16 pm, giving 12 hours, 10 minutes of daylight (7:11 am and 7:21 pm in Saint John). It crosses the equator heading south for winter at 10:54 pm on September 22, marking the beginning of autumn.
The Moon is at first quarter on Sunday and passes near Saturn on Monday. Venus is at its brightest, or greatest illuminated extent, on Friday but low enough after sunset to be hidden by trees or houses. Jupiter sets at 10 pm so it is observed best in late twilight before it gets too low for a steady view. Saturn continues to give awesome views in the early evening and sets around midnight this week. Mars remains a bright orange beacon toward the south all evening. Mercury is at superior conjunction behind the Sun on Thursday. If you are in a dark clear sky 60-90 minutes before sunrise, look for a subtle pyramid of light angling up from the eastern horizon: the zodiacal light, caused by sunlight reflecting off dust along the ecliptic.
This weekend has the final RASC NB star party of the season at the Kouchibouguac Fall Festival on September 14/15, and there is public observing at the Irving Nature Park in Saint John on September 14 from 7:30 to 11 pm (cloud date September 15).
Questions? Contact Curt Nason.