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By Science Museum on

How To Clear Your Calendar (the Extreme Way).

The 29th of February, a Leap day, is coming up again. On this mysterious date 20- year- olds celebrate their fifth birthdays and so on. What has this got to do with this beautiful armilliary sphere , on display in The Science Museum, London?


Armiliary sphere by Sisson (credit: Science Museum)

The sphere was made in 1731 for Prince Frederick , son of George II,  who died before his father, hence he never came to the throne. Both he and Princess Augusta were interested in what we would now call science. They commissioned this instrument, which shows the planets circling the Sun, from Jonathan Sisson, a leading London instrument maker. The ‘horizon ring’, the horizontal ring  round the instrument, is engraved with the days of the year and the signs of the zodiac.

However, the first day of spring, or first point of Aries, is not marked as the 21st of March but the 10th. The  equinoxes, when the days are the same length as the nights, had been moving backwards ever since Julius Caesar set the calendar in 46BC. This was because the 29th of February, coming every 4 years, was too frequent. In 1752 , in the reign of George II, the UK moved to the new calendar, when the 29th Feb was not quite every four years. In a stroke we lost 11 days-the 3rd to the 13th of September.  The painting by Hogarth ”An Election Entertainment’ has suggested there may have been riots over this loss, but there is no evidence.

 In some ways the UK was a conservative country and we were slow to make the change to the new calendar. In the 21st century precision timing rules our seasons.

One comment on “How To Clear Your Calendar (the Extreme Way).

  1. I celebrated my 5th Birthday, at the age of 20, on the 29th of Feb 2000. However had I been turning 20 in the year 2100 or 1900 I would not have had a birthday. I think these subtle exceptions to exeptions are the changes made to the calendar mentioned in the article: a year with 00 on the end has no Feb 29th, but a year with 000 does. I was lucky I didn’t have to wait 8 years for my birthday!

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