Letter to the Editor: On being up-to-date

To the Editor:

I was interested to see the article about how Ephraim Williams’ birthday had changed day and year because of the change in the mid-18th century from the Julian calendar (known as Old Style, or O.S.) to the Gregorian calendar (known as New Style, or N.S.) (“Invest-eph-gating a problem of birthdates,” March 11, 2015). I teach about the consequences of such changes in both my courses this semester.

In Astronomy/History of Science/Leadership 340: Great Astronomers and their Original Publications, we discuss the discovery of the four brightest moons of Jupiter by Galileo in early 1610 and independently by Simon Marius in late 1609. But Marius really wrote down his discovery a day after Galileo did – something that becomes clear once you add 10 days to Marius’ O.S. date in Protestant northern Europe to put it in the N.S. date scheme used by Galileo. Galileo was furious at Marius. Nonetheless, it was Marius who provided the names – Io, Europa, Ganymede, and Callisto – that we still use, and Thursday’s paper brought us news that scientists using a NASA spacecraft have discovered that Ganymede has a subsurface ocean – perhaps an indication of the presence of life.

In Astronomy 104: The Milky Way Galaxy and the Universe Beyond,  a survey of galaxies and cosmology, we looked at a photograph of the page of the Washington family Bible in which George was recorded as being born on February 11, 1731/2. We now use February 22 as his birthday. Apparently, residents of the colonies back then knew that dates in February (actually January through the vernal equinox) were likely to be considered in a later year, once the beginning of a year was changed from the equinox to January 1.

Jay Pasachoff

Field Memorial Professor of Astronomy

Director, Hopkins Observatory