February 29, 2024: Today is leap day, an additional day added to the year to keep the seasons aligned to the civil calendar.
by Jeffrey L. Hunt
Chicago, Illinois: Sunrise, 6:26 a.m. CST; Sunset, 5:41 p.m. CST. Check local sources for sunrise and sunset times. Times are calculated by the US Naval Observatory’s MICA computer program.
At Chicago’s latitude, daylight lasts 11 hours, 15 minutes, gaining 74 minutes during the month.
Leap Day
Today is leap day. It resets the seasons to the calendar. With this calendar readjustment, the equinox occurs on March 19th at 10:06 p.m. CDT. With cultural interests in keeping the seasons aligned with the months, the adjustment is needed. For example, the American Fourth of July celebration would eventually occur during the winter months without a realignment every four years.
A leap day is needed because Earth revolves around the sun, compared to the equinox, every 365.242 days. Every four years the fraction of a day nearly adds to a complete day.
A study of the modern calendar shows a messy past. Civil calendars were initially centered on the moon’s phases, but with a lunation lasting 29.5 days, the fraction of a day was split among months lasting 29 or 30 days. With 12 lunar months in a year, the length was only 354 days. Every three years an entire month was added to attempt to keep the calendar matched with the seasons.
Julian Calendar
In 46 BC, Julius Caesar reformed the calendar that has his name. He instituted a 365-day-year calendar, spreading the additional days from the lunar calendar among months of the year. Every four years a leap day was added to reset the date of the equinox and to keep the seasons from drifting too far from their traditional calendar locations. The year was defined to be exactly 365.25 days.
To fully reset the seasons to the calendar, Caesar added three extra months, making that year 445 days long.
However, the leap days did not fully reset the calendar. The Julian year is 11 minutes, 14 seconds longer than the year between successive spring equinoxes. During the next 370 years, the equinox date slipped four days since Caesar’s reform. By 1582, the small difference accumulated another 10 days.
Pope Gregory’s Reform
Pope Gregory XIII instituted another reform. The year’s length was set to 365.2425 days, only 53 seconds longer than the year defined by the spring equinox interval. As a result, ten days were removed from the calendar. October 4th was followed by October 15th. Catholic countries immediately changed their calendars. England and the American colonies did not make the change until 1752. Russia joined the Gregorian system in 1918.
Leap years still occur every four years and leap days are only added to century years divisible by 400. Another adjustment was made so that years 4,000, 8,000, and 12,000 are common years, without a leap day. The error is only one day in 20,000 years.
To find a date in history, Julian dates, successive days since January 1, 4713 BC are used. For example. George Washington’s birthday is noted on modern calendars as February 22, 1732, but a calendar in the Colonies read February 11, 1731.
Happy Leap Day!
Summaries of Current Sky Events
Summary for Venus as a Morning Star, 2023-24
Here is today’s planet forecast:
Morning Sky
Moon, Spica
One hour before sunrise, the gibbous moon (80% illuminated) is nearly one-third of the way from the horizon to overhead in the southwestern sky. It is 9.9° to the upper left of Spica.
The moon is approaching the Scorpion’s claws. This morning the lunar orb is over 10° to the lower right of Zubenelgenubi, the southern pincer.
Venus and Mars
Farther eastward, Venus and Mars are low in the east-southeast. Venus steps eastward faster than Mars. Earth’s Twin planet passed the Red Planet a week ago.
At 35 minutes before sunrise, brilliant Venus is over 5° above the horizon. Find a location with a clear view toward the east-southeast. Dimmer Mars is 3.4° to Venus’ upper right.
Wait another five minutes when the two planets are a little higher to see them through a binocular.
Saturn is west of the sun, rising at about the same time as the central star, so that it is invisible.
Evening Sky
Mercury is east of the sun, setting a few minutes after sunset. It is heading toward its best evening appearance of the year next month for northern hemisphere sky watchers.
Jupiter
An hour after sunset, bright Jupiter is nearly halfway up in the west-southwest. It is passing between Hamal, 11.4° to the planet’s upper right, and Menkar, 12.1° to the lower left. The Jovian Giant continues to slowly move eastward against the starfield.
Use a binocular to spot up to four of Jupiter’s largest moons that appear starlike on either side of the planet. Then look above the bright planet to explore the Pleiades and Hyades star clusters. Notice the contrasting star colors between the two stellar bundles.
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