The Sky This Week
U.S. Naval Observatory's Weekly Blog

"And the Seasons, They Go Round and Round"

by Geoff Chester, USNO Public Affairs
02 February 2021
Aldebaran, The Hyades, & The Pleiades, imaged from Mollusk, Virginia on 2019 December 31
Aldebaran, The Hyades, & The Pleiades, imaged from Mollusk, Virginia on 2019 December 31
with a Canon EOS Rebel SL2 DSLR, 24mm EF-S lens @ f/4, and an Omegon Mini Track LX2 mechanical star tracker.

The Moon brightens the early morning skies this week, coursing her way through the springtime constellations before ending the week among the rising stars of summer.  Last Quarter occurs on the 4th at 12:37 pm Eastern Standard Time.  Early risers will find Luna to the northeast of the bright star Spica on the morning of the 3rd.  On the 6th she will be just over four degrees north of Antares, the ruddy star that marks the “heart” of Scorpius, the Scorpion.

As I write this snow is falling outside of my window, and reports from rural Pennsylvania indicate that the rodent prognosticator of all things climatic has apparently seen its shadow, thus indicating six more weeks of winter.  This should not come as a big surprise to anyone since a simple glance at the calendar shows that the vernal equinox falls 45 days from today, just over six weeks away.  From an astronomical point of view, boreal winter is actually our shortest season, with a duration of just under 89 days.  This means that summer is our longest season with a length of just under 94 days.  The reason for the shorter duration of winter is that it begins shortly before Earth reaches perihelion, its closest point to the Sun.  This means that our planet is traveling at its fastest orbital speed during winter; the opposite is true in summer.  In fact, the length of summer is currently slowly increasing, while winter is gradually growing shorter.  This is caused by long-term variations in a number of Earth’s motions; the perihelion point drifts around the ecliptic about once in 111,000 years, while the equinoxes precess once in a bit over 25,000 years.  The net effect is to slowly change the lengths of the seasons in a periodic fashion with a period of around 21,000 years.  At present the extremes of summer and winter will be reached around the year 3500 CE.  Spring and fall will have the same length around the year 3850.  If you love winter you will have to wait awhile before it becomes the longest season.  That won’t occur until sometime around the year 13,500.  

The discussion above makes certain assumptions in the length of the mean solar day and the expected effects of long-term variations in Earth’s orbital eccentricity and axial tilt.  Known as Milankovich Cycles, these are very long-term variations that play out over hundreds of thousands of years and are caused by perturbations of the Sun, Moon, and (primarily) the planets Jupiter and Venus.  Ironically, we know more about the future effects of the Milankovich Cycles than we do about the Earth’s rotation over the long term.

While the seasons come and go, one thing that we can be certain of right now is that when you go outside at this time of year you will be greeted by the bright stars of the Great Winter Circle.  These bright stars and constellations fittingly grace the long nights of winter as they have for most of recorded human history.  The centerpiece of this show is the constellation of Orin, highlighted by his distinctive “belt stars” and the bright stars Betelgeuse and Rigel.  Over the past few weeks we have highlighted some of the surrounding bright stars, and this week we’ll look at Aldebaran, brightest star in Taurus, the Bull.  From a dark site this red-hued star seems to glimmer at the end of one tine of a V-shaped group of faint stars known as the Hyades.  This group is the closest galactic star cluster to Earth, a bit over 150 light-years away.  While Aldebaran appears to be a part of the cluster, it is about 85 light-years closer to us.  Its ruddy tint tells us that it is an evolved “red giant” star that has exhausted the supply of fusible hydrogen in its core.  Aldebaran gives us a “preview” of the fate of our Sun some 2 billion years from now. 

Ruddy Mars is now the only bright planet visible in the overnight hours.  Mercury, Venus, Jupiter, and Saturn are all too close to the Sun to be easily seen.  The red planet is moving steadily through the faint constellation of Aries, the Ram, toward Taurus, where he will encounter Aldebaran around the time of the spring equinox.

 
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