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Every Four Years. Almost...

by Geoff Chester, USNO Public Affairs | 27 February 2024

by Geoff Chester, USNO Public Affairs | 27 February 2024

Jupiter and Io, imaged 2024 February 21, 00:12.5 UT at the U.S. Naval Observatory
with the 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor
and a ZWO ASI183MC CMOS color imager

The Moon moves into the morning sky this week, waning to her crescent phase as she moves through the rising stars of the summer sky.  Last Quarter occurs on March 3rd at 10:23 am Eastern Standard Time.  Early risers that morning will find Luna less than a degree southeast of the bright star Antares in Scorpius in the southeastern sky.  

2024 is a Leap Year, so February gets one extra day tacked on before the next month falls.  Leap days are necessary to keep our calendar more or less in synch with the Sun and the dates of the equinoxes and solstices.  The problem is that the Earth takes just over 365 days to complete one orbit around the Sun, but that “extra” time rapidly accumulates, throwing a 365 day calendar rapidly out of synch with the sky.  

Roman emperor Julius Caesar addressed this problem in the year 46BCE by proposing a calendar with an extra day added to February every four years.  This produced a calendar year of 365.25 days, but the Earth’s annual trip around the Sun took 365.2422 days, making the Julian Calendar about 11 minutes longer than the “true” year.  This made the year agree somewhat better with the cycle of the seasons, but after just 128 years the difference between the date of the vernal equinox and the calendar grew by a full day.

Our current calendar is based on the reform promoted by Pope Gregory XIII in 1582, which dropped three leap years in a 400 year cycle.  This produces a calendar year of 365.2425 days, which is still some 11 seconds longer than the actual year, but the error won’t accumulate to a full day until sometime in the next millennium.  If we were to drop an additional leap year every 4,000 years, we would have a calendar that closely matches the duration of Earth’s actual annual trip around the Sun.  

The March campaign for the Globe at Night citizen science program begins on the evening of the 1st and runs through the 10th.  This program, established as part of the International Year of Astronomy 2009, encourages people to look up at the night sky and determine the faintest stars that they can see from a given location.  Over the course of 14 years, the program has amassed over 200,000 observations from 180 countries.  You can add your observations to the total by simply locating the bright constellation of Orion, the Hunter, and using the report form on the Globe at Night web app.

Fortunately, finding Orion is quite easy, even for city-bound skywatchers.  He is on the meridian at 7:30 pm and spends the early evening as a fixture in the southwestern sky.  His distinctive outline boasts two first magnitude stars and five luminaries of second magnitude.  Of his two brightest stars, one is linked to a constellation that will never be seen in the same sky as Orion.

In Greek mythology, Orion was the son of the Gorgon Euryale and Neptune who grew to be a very skilled and powerful hunter.  Among his many adventures, he was called to the island of Chios by the king Oenopion rid the land of ferocious animals that were terrorizing the residents.  Quickly dispatching the beasts, he then boasted that he would slay every animal on Earth.  This naturally infuriated the Earth goddess Gaia, who sent a lowly scorpion to put the Hunter in his proper place.  The scorpion succeeded in killing Orion, after which Gaia placed both combatants in the sky, but in such a way that they would never both be visible together at the same time.  Orion’s bright star Betelgeuse and Scorpius’ counterpart Antares are both red supergiant stars; one rises as the other sets, never sharing the same limelight.

Jupiter continues to settle in the western sky with each passing evening.   You can still get a good view of the giant planet beginning shortly after sunset, but if you want to get a clear view of him through the telescope, try to view him before 8:30 pm to minimize turbulence in Earth’s atmosphere.

Venus is now wallowing in the gathering morning twilight.  You will find the dazzling planet very low in the southeast about half an hour before sunrise.  Venus will linger in this general area for several more weeks, gradually drifting northward over that time.  If you have binoculars, try to spot the fainter glow of Mars just over 2 degrees to the right of Venus over the course of the week.


Commander, Naval Meteorology and Oceanography Command | 1100 Balch Blvd. | Stennis Space Center, Mississippi 39529

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