Fred Barrett | Feb 06, 2019
I always try to give you a head’s up (get it?) whenever the Zodiacal light can be viewed in its entire splendor in the night sky. This month, we’re in luck! Its cone-shaped radiance will be best viewable from mid-February through early March after sunset. Let me describe to you what the Zodiacal light is, and then we can understand the ‘why’ of the best times to view it: It is a diffuse cone or triangle of light that rises up from the western horizon after sunset. The Zodiacal cloud is the debris disk of our solar system and its radiance is caused by sunlight scattering and reflecting off the particles in that cloud. The disk of dust ranges through the inner solar system and extends out from the Sun to the orbit of Jupiter. The debris mainly originates from the boiling off of materials from comets and material thrown off by asteroid collisions. The dust particles have a size range from about a few micrometers to a few centimeters. The debris mainly orbits in the ecliptic plane – the plane in which the planets travel around the solar system. Thus, the best time to observe its glow is when the axis of the Earth’s rotation is at a maximum to the ecliptic plane - perpendicular to the horizon. This occurs around the time of the spring and autumnal equinoxes. At the spring equinox (February-March), it will appear about an hour after sunset on the western horizon. At the time of the autumnal equinox (September-October), it will present itself before dawn on the eastern horizon. The Zodiacal light has been observed for a long time by many civilizations. In the Bible, it is written that ancient astronomers called it the ‘wings of the morning’ because when it appeared in the Fall at sunrise, it signaled the dawn. The best time to view the Zodiacal light is on a late winter evening. Find yourself a nice dark site when the Moon isn’t around. February 15 to March 7 is a good interval to observe. Dress warmly (need I say), get your eyes dark adjusted and look to the west. You don’t need binoculars!
Mercury is a very bright sight in the west southwest after sunset. On the 15th it’s 5 degrees above the horizon at a brightness magnitude of -1.1. By the 26th, it shines at a magnitude of -0.5 at a height of 11 degrees above the horizon. Magnitude is a measure of brightness as observed from Earth. It is a logarithmic scale which means that a difference of 1 in magnitude means a change in brightness by a factor of 2.512. Note that magnitude in the positive range is much dimmer than a magnitude in the negative. The brightest star in our night sky is Sirius. It has a magnitude of -1.46. So Mercury’s magnitude of -0.5 on the 26th is indeed bright.
Mars is noticeable in the constellation Aries in the west southwest. If you are up early just before dawn, Venus, Jupiter and Saturn are all in the southeast.
Comet Wirtanen is still a binocular object near Ursa Major.
All Month: Zodiacal Light.
February 12: First quarter Moon.
February 18: Venus passes 1.1 degree north of Saturn, about 45 minutes before sunrise in the southeast.
February 19: The Moon is at perigee (closest): 221,681 miles. Full Moon occurs. This Moon is called the Full Snow Moon for obvious reasons. It was referred to by first nations as the Full Hunger Moon – very difficult to hunt for food.
February 26: Last quarter Moon.
That’s a wrap. Keep looking up!
“The Beginner’s Observing Guide by Leo Enright is an invaluable companion for adventures in the sky. It also contains useful star charts. It can be ordered from the Royal Astronomical Society of Canada at www.rasc.ca/publications. A subscription to our very own excellent Canadian astronomy magazine, “SkyNews”, can be arranged at the RASC website as well.