Assume an event occurs the third Thursday of March, the 22nd.  Since the 22nd would fall on different days in future years, would the true anniversary of the event be the third Thursday or the 22nd?

This is an interesting question and one that gets into the semantics of how we record time.  To confuse matters even more, we have a leap year every 4 years, because the Earth's orbit around the Sun is 365 and 1/4 days (it actually gets even trickier).  Keeping true to the meaning of anniversary, one cycle around the Sun, I would say using the number (the 22nd in this case) is more accurate as the fractions of days will be more easily absorbed, especially when it is a leap year.

Just to confuse the situation even more, a year is actually 365.24, and by doing leap year, we actually accumulate 0.000125 days per year, meaning over a period of 8,000 years, we will lose a single day to account for this.  To add further to this mess, since we are slowly losing the Moon (it is drifting away at a rate of 4 cm/year), a day on Earth is actually getting longer.  In fact, every 5,000 years, we gain a second on the day (which means the definition of a day is changing), and over time, as the Moon gets further out, this will rapidly increase.

Time keeping is a tricky business!

I hope this helps (more than it confuses!)  


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Brad Tucker


I'm happy to answer any general questions about Astronomy, Astrophysics, and Cosmology. I'm also happy to take general, specific, and detailed questions related to supernovae, Baryonic Acoustic Oscillations, the Cosmic Microwave Background, dark matter, dark energy, and the Big Bang Theory. I'm also happy to chat about Astronomy/Astrophysics education and careers, and philosophy and science.


I am a professional research astronomer/astrophysicist/cosmologist. My research focuses on studying supernovae and using them to measure the properties of the Universe, such as how fast it is growing and what it is made of. I also frequently give talks to school groups and the public, and am a regular guest on various radio stations.

Current Research Fellow at Mt. Stromlo Observatory, the Australian National University, and in the Department of Astronomy, University of California, Berkeley.

Lots of journals, including the Astrophysical Journal, the Astronomical Journal, and Nature. I am currently in the middle of writing my first popular book.

B.A. Philosophy, University of Notre Dame, Notre Dame, IN USA B.A. Theology, University of Notre Dame, Notre Dame, IN USA B.Sc. Physics, University of Notre Dame, Notre Dame, IN USA Ph.D. Astrophysics, Mt. Stromlo Observatory, the Australian National University, Canberra, ACT, Australia

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