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The Sky This Week, 2020 February 25 - March 3

Taking a leap (year).
Crescent Moon and Venus, 2018 April 17, imaged by Dr. Marc Murison at USNO's Flagstaff (AZ) station.
Crescent Moon and Venus, 2018 April 17
Imaged by Dr. Marc Murison at USNO's Flagstaff (AZ) station
with a Canon PowerShot G3 X.

The Moon waxes in the evening sky this week, climbing northward along the ecliptic toward the bright stars of the Great Winter Circle.  First Quarter occurs on March 2nd at 2:57 pm Eastern Standard Time.  Luna starts the week near the dazzling glow of Venus, passing closest to the planet on the evening of the 27th.  

2020 is one of the years in our calendar in which February has 29 days.  We call these events “Leap Years”, and most people think that this is an event that happens every four years.  We need to make this periodic adjustment due to the fact that the time it takes the earth to complete one orbit around the Sun isn’t exactly 365 days.  The true duration of the so-called Tropical Year is 365.2422 days, so to keep the calendar in synch with the astronomical seasons an extra day is periodically required.  The origins of this practice dates back to the rule of Julius Caesar, when the four-year leap year cycle was introduced in his calendar reform in the year 46 BCE.  While the Julian Calendar gave a fair approximation of the true year, it was 11 minutes longer than the tropical year, resulting in an accumulated error of one day after 128 years.  By the 16th Century CE this error amounted to 10 days, wreaking havoc in religious calendars that fixed feast dates by the occurrence of the vernal equinox.  A number of calendar reform schemes were proposed during the late 15th and 16th Centuries, but it was ultimately scholars working for Pope Gregory XIII that came up with the system that we still use today.  In 1582 he promulgated, through the Papal Bull “Inter Gravissimus”, his reformed calendar.  Rather than a four year leap year cycle, Gregory’s calendar counted 97 leap days in a 400 year cycle.  The three “ordinary” years in each cycle were years ending in “00” that were not evenly divisible by 400.  Under this scheme the years 1700, 1800, and 1900 were ordinary years, while the year 2000 was a leap year.  The length of a mean Gregorian year is 365.2425 days, so it’s still slightly longer (by an average of about 11 seconds) than the tropical year.  This will result in a one day error in a bit over 3000 years, but due to slow changes in the Earth’s orbit and the gradual slowing of the Earth’s rotation we probably won’t need to reform the current calendar for another 7000 years.  However, if we do reform the calendar, we will probably use the scheme promoted by Sir John Herschel in the 19th Century.  He proposed a 4000 year cycle that had 969 leap days, with millennial years evenly divisible by 4000 counted as ordinary years instead of leap years.  Fortunately we have plenty of time to contemplate this issue!

We are also entering the time of the year when the daily change in daylight reaches its peak rate.  As we enter March, the length of day increases by about three minutes per day.  Between now and the equinox on March 20th we’ll add a full hour of daylight to the 11 hours we’re experiencing now.

The winter stars still dominate the early evening hours, but by late evening the stars of Leo and Ursa Major herald the imminent arrival of spring.  If you follow the “arc” of the “handle” of the Big Dipper asterism, which is now prominently placed in the northeast, you’ll find a furiously flickering bright yellow-tinted star rising at around 9:00 pm.  This is the star Arcturus, the brightest star in the northern hemisphere of the sky, and the star that I have always associated with warmer evenings as winter loses its grip on the landscape.

Venus continues to climb higher in the west as she drives steadily northeastward along the ecliptic.  By the end of the week she is poised to move into the constellation of Aires, the Ram.  Over the course of the next month she will draw a bead on the Pleiades star cluster, passing through the cluster in early April.

Three bright planets may be found in morning twilight among the rising stars of Sagittarius.  Look low in the southeast for the ruddy glimmer of Mars, the bright cream-hued glow of Jupiter, and the yellow tint of Saturn.  Early in the week the distance between Mars and Saturn spans about 20 degrees, with Jupiter perched between them.  Mars will move eastward with each passing morning, and by next week the line of planets will span 16 degrees.  Over the course of the next few weeks Mars will catch up with Old Jove and pass the giant planet on March 20th.

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