A Full Sap Moon as the Sun Brings Spring, plus Morning Planet Parade and Evening Zodiacal Light!
Happy Vernal Equinox!
Two few minutes before 6 pm Eastern Daylight Time on Wednesday, 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 bowls — the Northern and Southern hemispheres. Meanwhile, as Earth orbits around 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 Celestial Equator and the Ecliptic 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’ve referring to above is the one that causes new stars to appear each season, and is NOT the one that carries the sun across the sky every day. The first case is due to the Earth’s year-long orbit and the second motion is due to the Earth’s daily rotation.)
The sun’s eastward motion along the ecliptic circle covers 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 daily sun time means warmer air and longer daylight hours! At the same time, folks in the Southern hemisphere have to accept shorter, colder days and longer nights (Warmly dressed astronomers don’t mind 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 also offer better chances to see the aurorae at high northern and southern latitudes. 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 is tilted neither towards nor away from the sun, so the two “magnets” aren’t as parallel, reducing Earth’s ability to deflect the sun’s field and the charged particles that trigger aurorae in our upper atmosphere.
The Moon and Planets
For most of this week, the moon will remain in view in the evening sky. On Sunday night, it will be a bright gibbous (more than half-illuminated) orb between Cancer (the Crab) and Leo (the Lion). On Monday night, the moon will land less than two finger widths to the left of Leo’s brightest star, Regulus.
On Wednesday night the moon will reach its full phase. Although technically it occurs a few hours past the equinox, this is the final full moon of winter. The March full moon, known as the Worm Moon, Crow Moon, Sap Moon or Lenten Moon, always shines in or near the stars of Leo or Virgo (the Maiden). Full moons always rise in the east as the sun sets, and set in the west at sunrise. When fully illuminated, the moon’s geology is enhanced, especially the contrast between the ancient cratered highlands and the younger, darker, smoother maria. 2019 marks the 50th anniversary of the first lunar landings by humans.
After Wednesday, the moon will begin to rise in late evening and wane in phase. From Thursday to Sunday, it will traverse the stars of Virgo and then Libra (the Scales).
Mars will continue to be an easy planet to see every evening this week, but only for a couple of hours after dusk. By midnight, Mars will set in the west. Once the sky has darkened, look for Mars’ medium-bright, reddish pinpoint of light less than halfway up the western sky. Mars has been slowly shrinking in size and brightness as we increase our distance from it little-by-little. Distant Uranus is situated two fist diameters below Mars — but it’s too low for observing nowadays.
The eastern pre-dawn sky continues to host a spectacular parade of three bright planets. Bright Jupiter will rise first, at about 2:30 am local time. By 7 am, it should still be visible in the southern sky. Yellowish Saturn, will rise at about 4:30 am local time and will become lost in the southeastern twilight before 7 am.
Our sister planet Venus, now markedly closer to the sun, is starting to become engulfed in the dawn twilight. Look for Venus’ as a bright beacon sitting quite low in the east-southeastern dawn sky from 6 am local time until sunrise. In a telescope, Venus will exhibit a gibbous (more than half-illuminated) phase. If you have trouble seeing Saturn, search about midway between Jupiter and Venus.
The Brightest Stars
During full moon periods, only the brightest stars can still be spotted with unaided eyes. At this time of year, most of those stars are the ones that form the Winter Hexagon asterism. Start by finding the extremely bright star Sirius sitting low in the southern evening sky at 9 pm local time. From there, look for bluish Rigel sitting 2.5 fist diameters to Sirius’ upper right, then look well above Rigel for warm-tinted Aldebaran, and continue to Aldebaran’s upper left to reach yellowish Capella at the top of the asterism. Now descend on the hexagon’s left side. The bright matched pair of stars Castor and Pollux is three fist diameters to the lower left of Capella. Finally, bright white Procyon is well below those twins — roughly between them and Sirius.
The only other bright star is Regulus in Leo. That white star sits about 3.7 fit diameters to the left (east ) of Procyon.
Evening Zodiacal Light
For about half an hour after dusk between today and the new moon on April 5, look west-southwest for a broad wedge of faint light rising from the horizon and centered on the ecliptic. This is the zodiacal light — reflected sunlight from interplanetary particles of matter concentrated in the plane of the solar system. The glow will be centred on the horizon directly below Mars. Try to observe from a location without light pollution, and don’t confuse the zodiacal light with the brighter Milky Way to the northwest.
Astronomy Skylights for the week of March 17th, 2019 by Chris Vaughan.
Keep looking up, and enjoy the sky when you do. I love questions and requests — so, send me some!