Astronomy Picture of the Day [1]Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. 2020 February 29 [2]See Explanation. Clicking on the picture will download the highest resolution version available. Julius Caesar and Leap Days Image Credit & [3]License: [4]Classical Numismatic Group, Inc., [5]Wikimedia Explanation: [6]In 46 BC Julius Caesar reformed the calendar system. Based on advice by astronomer [7]Sosigenes of Alexandria, the Julian calendar included one [8]leap day every four years to account for the fact that an Earth year is slightly more than 365 days long. In modern terms, the time it takes for the planet to orbit the Sun once is 365.24219 mean solar days. So if calendar years contained exactly 365 days they would drift from the Earth's year by about 1 day every 4 years and eventually July (named for [9]Julius Caesar himself) would occur during the northern hemisphere winter. By adopting a leap year with an extra day every four years, the Julian calendar year would drift much less. In 1582 Pope Gregory XIII provided the further fine-tuning that leap days should not occur in years ending in 00, unless divisible by 400. This [10]Gregorian Calendar system is the one in wide use today. [11]Of course, tidal friction in the Earth-Moon system slows Earth's rotation and gradually lengthens the day by about 14 milliseconds per century. That means that [12]leap days like today will not be necessary ... about 4 million years from now. Tomorrow's picture: a hole in Mars __________________________________________________________________ [13]< | [14]Archive | [15]Submissions | [16]Index | [17]Search | [18]Calendar | [19]RSS | [20]Education | [21]About APOD | [22]Discuss | [23]> __________________________________________________________________ Authors & editors: [24]Robert Nemiroff ([25]MTU) & [26]Jerry Bonnell ([27]UMCP) NASA Official: Phillip Newman [28]Specific rights apply. [29]NASA Web Privacy Policy and Important Notices A service of: [30]ASD at [31]NASA / [32]GSFC & [33]Michigan Tech. U. References 1. https://apod.nasa.gov/apod/archivepix.html 2. https://apod.nasa.gov/apod/image/2002/CaesarCoin_Wikipedia_960.jpg 3. https://creativecommons.org/licenses/by-sa/3.0/deed.en 4. http://www.cngcoins.com/ 5. https://commons.wikimedia.org/wiki/File:RSC_0022_-_transparent_background.png 6. https://uh.edu/engines/epi2364.htm 7. https://en.wikipedia.org/wiki/Sosigenes_of_Alexandria 8. http://scienceworld.wolfram.com/astronomy/LeapYear.html 9. http://www.literaturepage.com/read/shakespeare_juliuscaesar.html 10. http://galileo.rice.edu/chron/gregorian.html 11. https://apod.nasa.gov/apod/ap171007.html 12. https://scienceblogs.com/startswithabang/2012/02/29/the-physics-of-leap-day 13. https://apod.nasa.gov/apod/ap200228.html 14. https://apod.nasa.gov/apod/archivepix.html 15. https://apod.nasa.gov/apod/lib/apsubmit2015.html 16. https://apod.nasa.gov/apod/lib/aptree.html 17. https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search 18. https://apod.nasa.gov/apod/calendar/allyears.html 19. https://apod.nasa.gov/apod.rss 20. https://apod.nasa.gov/apod/lib/edlinks.html 21. https://apod.nasa.gov/apod/lib/about_apod.html 22. http://asterisk.apod.com/discuss_apod.php?date=200229 23. https://apod.nasa.gov/apod/ap200301.html 24. http://www.phy.mtu.edu/faculty/Nemiroff.html 25. http://www.phy.mtu.edu/ 26. https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html 27. http://www.astro.umd.edu/ 28. https://apod.nasa.gov/apod/lib/about_apod.html#srapply 29. https://www.nasa.gov/about/highlights/HP_Privacy.html 30. https://astrophysics.gsfc.nasa.gov/ 31. https://www.nasa.gov/ 32. https://www.nasa.gov/centers/goddard/ 33. http://www.mtu.edu/