Easter Sunday has an astronomical connection. In 325 AD the Council of Nicaea determined that Easter should be observed on the Sunday following the first full moon that occurs after the vernal equinox, officially known as the Paschal Full Moon. The council fixed the equinox at March 21, but the timing actually varies a little bit, and even occurs a full day early on Leap Years. This year, it was on March 20, and the full moon was March 31 (yesterday).
Some believe that the full moon allowed for safer travelling by religious pilgrims of the time. This year’s April 1 Easter Sunday is on the early side, but the earliest that Easter can ever occur is March 22, which last happened in 1818, and won’t happen again until 2285! If a full moon happens just before a late arriving equinox, Easter gets delayed by a full lunar month plus up to six days — on April 25. This will happen in 2038.
End of Tiangong-1
You may have heard that Tiangong-1, China’s abandoned space station, is about to re-enter Earth’s atmosphere and fall to Earth. Any spacecraft in Low Earth Orbit (LEO), including the ISS, is subject to a tiny amount of friction by the Earth’s outer atmosphere. The friction slows the orbit, causing it to descend. Without periodic boosts in speed, the object will eventually fall to Earth, fully or partially burning up during re-entry. And anything left over will reach the ground.
Most of the time, engineers have some say over when and where a craft will land, usually directing it into the vast unpopulated Pacific Ocean. In the case of Tiangong-1, China has no control over the station, but they DO have a good idea of its orbit. At present, it is predicted to fall sometime today or tomorrow (Monday) somewhere between 42.7° N and 42.7° S latitude.
The Heavens-Above website has a live tracking tool here. If you input your own location, it will tailor the information to you. Keep an eye on the Height Above Ground value. When that drops below 100 km, the end is close. Its fiery re-entry should be easy to see at night, and possibly in daytime, too — if you know where to look. Let’s hope it misses any populated areas!
This representation of Tiangong-1′s decaying orbit is courtesy of Heavens-Above.com. The spacecraft is predicted to fall to Earth sometime on April 1–2, 2018. The debris field will be somewhere between the parallels of 42.7 degrees, likely into the Pacific Ocean.
Several prominent winter asterisms persist into early April. During early evening the Winter Hexagon leans to the right over the southwestern horizon. Also known as the Winter Circle and Winter Football, it is composed of (moving anti-clockwise from the bottom left) bright white Sirius (Alpha Canis Majoris), blue-white Rigel (Beta Orionis), orange Aldebaran (Alpha Tauri), yellow Capella (Alpha Aurigae), white Castor and golden Pollux (Alpha and Beta Geminorum respectively), and white Procyon (Alpha Canis Minoris).
Sirius, Betelgeuse, and Procyon also form another asterism called the Winter Triangle. For mid-northern latitude observers, Sirius is always seen in the lower third of the sky, through a thicker blanket of Earth’s refracting atmosphere. This produces the strong twinkling and flashes of color the Dog Star is known for.
Turning around and looking the other way, the constellation of Leo strongly resembles the lion it’s named for. Already stalking the eastern sky at nightfall, this beast is an all-night constellation. Its heart is marked by its brightest star, white Regulus. Regulus also serves as the point on the backwards question mark asterism, also known as the Sickle, that forms the lion’s neck and head. Algieba (Gamma Leonis) is the medium-bright star that marks the lion’s throat, and is an easy double star in small telescopes.
Speaking of double stars, the dim constellation of Cancer (the Crab) sits nearly overhead in the southern sky during mid-evening. It contains quite a few nice multiple stars that make ideal binocular or telescope targets on moonlit nights. Phi (φ) Cancri is a very wide double easily seen in binoculars. Zeta (ζ) Cancri, also known as Tegmine, is a spectacular triple of yellow stars located 83 light-years from Earth. And, Iota (i) Cancri, or Zubanah, is a fantastic golden and blue pair visible in telescopes.
The Moon and Planets
After this weekend’s full moon, the moon spends this week rising later and later, and waning to its Last Quarter phase on Saturday morning. At that time, the right-angle between the Earth, moon, and sun will produce a half-lit moon, on the dawn (left-hand) side. When the gibbous moon rises just before 11 pm local time on Monday evening, it will be positioned a few finger widths to the upper right of Jupiter, in the constellation of Libra (the Scales). The pair will cross the sky together for the rest of the night, eventually moving to a point low in the southwestern pre-dawn sky on Tuesday morning.
In the southeastern pre-dawn sky on Saturday, the last quarter moon will form a picturesque linear grouping with Mars and Saturn. Reddish Mars will sit about a palm’s width to the lower left of the moon, with yellowish Saturn roughly midway between them. The trio will appear low over the horizon after 3 am local time. By dawn, they will be higher, and the moon will have shifted closer to Saturn. The trio will fit nicely into the field of view of binoculars, and make a nice photograph.
The long-awaited morning conjunction of reddish Mars and yellowish Saturn has arrived! Tomorrow morning (Monday), Mars will sit only a finger’s width below Saturn. Look for the two planets low over the southwestern horizon before dawn. From now until July, Mars will stretch its distance from Saturn. Then it will reverse course and head west again. (I’ll have plenty to say about Mars during the coming months.)
Extremely bright Jupiter is dominating the southern overnight sky now. After it rises in the east about 11 pm local time, don’t be surprised if it catches your eye out the window if you are out of bed during the wee hours! The mighty planet reaches its highest elevation (about three fist diameters) above the southern horizon around 4 am, and then drop into the southwestern sky just before sunrise.
Our views of Venus continue to get better and better. The extremely bright planet gleams over the western horizon for almost two hours after sunset.
Astronomy Skylights for this week (from April 1st, 2018) by Chris Vaughan. Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.