Happy Vernal Equinox!
At about 12:15 pm Eastern Daylight Time on Tuesday, our northern Spring, also known as the Vernal Equinox, officially begins! Here’s why…
The Celestial Equator is an imaginary circle around the sky that sits directly above the Earth’s equator. It divides the sky into two equal bowls — the Northern and Southern hemispheres. Meanwhile, as Earth orbits the sun, the sun appears to travel eastward through the distant stars, tracing out another circle called the ecliptic. Due to the 23.5° tilt of the Earth’s axis of rotation, the two circles are tipped with respect to each other. Think of them as two hula hoops with the same centre, Earth — but one is tilted so that they intersect at only two spots. (The motion of the sun that I’m referring to is the one that causes new stars to appear each season, and not the one that carries it across the sky every day. The former motion is due to the Earth’s year-long orbit and the latter motion is due to the Earth’s daily rotation.)
The sun’s eastward motion along the ecliptic circle traverses about one degree per day. At the precise moment of the Vernal Equinox, the sun is “stepping over” the equator (where the hula hoops cross) and its apparent motion is carrying it into the northern half of the sky. Six months from now, on the Autumnal Equinox, it will again cross the equator heading into the southern half of the sky.
This produces two interesting effects. Firstly, for the next six months, the sun will spend the majority of each day in our northern hemisphere sky, overhead of the lucky folks in North America, Europe, and Asia! More sun time means warmer air and longer daylight! At the same time, folks in the Southern hemisphere have to accept shorter, colder days and longer nights (Warmly dressed astronomers like long winter nights!). Secondly, on the day of the equinoxes, we experience about 12 hours each of daytime and night-time (it varies by latitude). This is where the word equinox (Latin for equal night) comes from.
The times around the equinoxes offer better chances to see the aurorae at high northern and southern latitudes, too. Just as two bar magnets lined up with their poles in the same direction repel one another strongly, the Earth’s magnetic field repels the sun’s field. At the equinoxes, the Earth’s axis tilts neither towards nor away from the sun, so the two “magnets” aren’t lined up as well, reducing Earth’s ability to deflect the sun’s field and the charged particles that trigger aurorae in our upper atmosphere.
Walking the Dogs’ stars
The night sky’s brightest star Sirius gleams in the evening during early spring every year. When the sky darkens about 8 pm local time, the star sits above the southern horizon, just to the lower left of Orion (the Hunter). For the rest of the evening, Sirius slowly descends as it passes into the southwestern sky before setting about 1 am local time.
Sirius, also known as the Dog Star, is the brightest star in Canis Major (the Greater Dog). The constellation genuinely resembles a wiener dog! Sirius sparkles at the dog’s collar. The pup’s head is formed by several medium-bright stars to Sirius’ upper left. These are near the limit of visibility in urban skies.
The rest of the dog’s body, composed of more easily visible stars, extends to the lower left (southeast) of Sirius. The dog is rearing up and facing west, as if he is begging Orion for a treat. About a fist’s diameter to the lower right of Sirius, we find the bright star Wezen, which marks the dog’s “bottom”. Wezen is a rare yellow supergiant star. One day it will explode in a supernova. The tip of the dog’s tail, marked by a modest star named Aludra, is found four finger widths to the lower left of Wezen. Four finger widths to the lower right of Wezen, a bright star named Adhara represents the dog’s rear legs. (Some representations include two dimmer stars for the rear paws.) Adhara is a hot blue giant star with a surface temperature of a whopping 21,000 K located about 34 light-years from the sun. It’s the brightest star in the sky when viewed in ultraviolet light, and it, too, is on the way to a supernova death.
The dog’s front legs are formed by the bright star Mirzam, which is located about a palm’s width to the lower right of Sirius. Mirzam, which means “the Herald” because it rises just before Sirius, is 60 times more luminous than Sirius. If it were located where Sirius is, instead of 500 light-years away, it would appear 15 times brighter than Venus!
In the heart of Canis Major, about four finger widths below Sirius, is a bright little cluster of stars designated Messier 41, sometimes called the Little Beehive Cluster. Binoculars should show it easily. The cluster, which is about 2300 light-years away, consists of several brighter golden stars and numerous fainter ones.
Canis Major is only one of Orion’s two hunting companions. The other, Canis Minor (the Smaller Dog), sits 30° (three fist diameters) to Orion’s left. This constellation is composed of only two stars — very bright white Procyon and dimmer Gomesa, which sits about four finger widths to Procyon’s upper right. Ironically, the constellation resembles a stick more than a dog! The two dogs might be hunting Lepus (the Rabbit), a constellation of modest stars that sit directly below Orion.
Sirius is so bright because it is about 25 times more luminous than our Sun, and only a mere 8.6 light-years away from Earth. Furthermore, it is heading towards us, and will brighten over the next millennia! Sirius has a tiny companion star, designated Sirius B, that some astronomers call the Pup. I prefer to call it the Flea!
Sirius is famous for exhibiting flashes of intense colour as it twinkles. This is because northern hemisphere observers usually see the star positioned low in the sky, when its very bright starlight is passing through a thicker blanket of air. The pockets of turbulence in our atmosphere that makes stars twinkle also work like tiny refracting prisms — splitting apart Sirius’ white starlight and randomly sending different colours (wavelengths) to our eyes.
The ancient Egyptians linked their calendar to the arrival of Sirius in the pre-dawn sky because it signaled the onset of the Nile floods around the beginning of summer. In China, Sirius is called Tiān Láng天狼, aka “the Celestial Wolf”. Many First Nations cultures saw a dog’s shape in these stars and called Sirius the Moon Dog Star (Inuit), the Wolf Star (Pawnee), and the Coyote Star. On the next clear evening, have a look at our bright neighbour!
The Moon and Planets
The moon starts this week as a very slim crescent lingering over the western horizon for a short while after sunset. Tonight (Sunday), very bright Venus will be positioned just four finger widths to the moon’s upper right, and much dimmer Mercury will be the four fingers beyond Venus. (After the moon moves away tomorrow, the two planets will remain there.)
For the rest of the week, the moon will wax and climb east, leaving the western twilight. On Friday evening, March 22, in the southwestern evening sky, the waxing crescent moon will pass just to the upper left of Aldebaran, the brightest star in Taurus (the Bull). Closest approach will occur around 7 pm EDT. While it won’t yet be fully dark in the GTA, binoculars will still reveal the star. Observers in northeastern Russia, northwestern North America, Greenland, Svalbard, most of Scandinavia, Great Britain, and Ireland can see the moon’s orbit carry it across Aldebaran. The star will wink out when the moon’s dark leading limb covers it, and re-appear from the opposite lit limb less than an hour later.
Saturday brings the moon to a position at right angles with Earth and the sun, producing its First Quarter phase. At first quarter, the moon rises at noon and sets about midnight, putting it nicely in the southern sky during the evening. The dates surrounding first quarter are perfect for seeing the dramatically shadowed terrain along the Terminator, the boundary separating the dark and lit hemispheres. The sun’s light is striking the moon at a shallow angle, stretching all the shadows and leaving even shallow craters in darkness. New vistas are illuminated every night! Use binoculars or a small telescope.
Extremely bright Venus continues its ascent from the evening twilight this week. You can look for its blaze of brightness well above the western horizon after sunset, until it sets at about 8:50 pm local time. Much dimmer Mercury is easy to see this week when using Venus as your guide. It will be sitting about four finger widths to the upper right of Venus tonight, and then begin to drop lower than Venus through the week. Viewed in a telescope the hot little planet will exhibit a waning crescent phase, while Venus will be nearly fully illuminated. The best viewing time for Mercury falls between 7:40 and 8:40 pm local time.
Extremely bright Jupiter will be rising just before midnight local time this week. It will reach its highest elevation (about three fist diameters) above the southern horizon by 5 am local time. And it will continue to catch your eye in the southwestern sky as you leave for school or work until close to sunrise.
Reddish Mars is dimmer than Jupiter, but it is steadily brightening as Earth closes our distance from it. Over the next five months, Mars will begin to outshine everything but the king of planets, the moon, and the bright star Vega. For this week, Mars will be rising at about 3 am local time and will appear over the southern horizon before dawn.
Mars has been steadily moving eastward towards yellowish Saturn. This week, the two planets will be only a generous palm’s width apart, with Mars to Saturn’s upper right. The ringed planet rises just before 3:30 am local time, putting it two fist diameters above the southeastern horizon just before the dawn sky begins to brighten. The teapot-shaped constellation of Sagittarius (the Archer) will sit directly below Saturn all year.
Astronomy Skylights for this week (from March 11th, 2018) by Chris Vaughan. Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.