Welcome, Indigo Night Sky watchers. This week, the Autumnal Equinox occurs on Thursday, the 22 at 10:21 a.m., EDT.

Our blog post is focused on the equinox, but will also mention a few celestial sights visible this week – if you are willing to stay up late enough.

In common usage, the word equinox refers to the two diurnal cycles in the course of a year (one in spring, one in fall) when the duration of daylight is approximately equal to the duration of night – 'equi' meaning 'equal,' and 'nox' meaning 'night.'  More precisely, the moment of the autumnal equinox is when the sun appears to cross the celestial equator, heading southward. Due to the sun's direction of motion at this time, the event is also called the southward equinox, in contrast to the northward equinox, which occurs at the start of spring. The celestial equator can be understood as the Earth's equator projected outward to become the equator of the celestial sphere, the massive, hollow ball with the Earth at its center that serves as a useful model for mapping and predicting the apparent motion of 'fixed' stars as seen from our home planet. On the days of equinox, the sun rises directly in the east and sets directly in the west. Prior to the autumnal equinox, the sun has been rising and setting more to the north, resulting in summer for the northern hemisphere, and winter for the southern hemisphere. From the autumnal equinox forward, the sun will rise and set more and more to the south, bringing winter north of the equator, and summer to the south.

Due to its occurrence at the time of harvest in the northern hemisphere, the autumnal equinox has often been associated with festivals celebrating the fruits of agricultural labor, but my favorite association with this special day is that the French First Republic that was proclaimed (and the monarchy abolished) on September 21, 1792. Thus, the following day, September 22, 1792, which was the day of the autumn equinox that year (exactly 224 years prior to this year's autumnal equinox), became the first day of the 'Republican Era' in France.

This alignment of civic life to astronomy resonates for me in at least three meaningful ways. The first is that it represents the French Republic's commitment to science and rational thought as appropriate bases for sound governance. Understanding of astronomy had been growing at a rapid pace over the previous hundred years, largely due to the contributions of Isaac Newton at the start of the 18th century. By tying the start of the Republic to the equinox, the French made a bold statement of their faith in reason. The second powerful meaning in this equinoctial start to the Republic is that is demonstrates France's decisive turning away from the Sun Kings by letting the start of the nations's new republican day be determined by the movements of the real sun, instead.  And finally, the choice of the equinox for the first day of the French Republic highlights the importance of égalité – 'equality' – as one of the three fundamental concepts driving the revolution (liberté, égalité, fraternité was the rallying cry: 'liberty, equality, fraternity'). To begin the new national government on the day of equal light and darkness was a profoundly meaningful choice, and although the Republican Calendar did not endure, I always enjoy recalling it as the autumnal equinox marks the onset of fall.


A rare and beautiful automata work, this Republican Clock has both Republican and Gregorian dials and others that indicate the age and phase of the moon, time of sunrise and sunset, equation of time, world time and signs of the zodiac.  The clock synced with the Republican Calendar, in use between 1793 and 1805, designed in part to remove all religious and royalist influences.

As for celestial objects visible with the naked eye this week, the best bets are between about 10:30 p.m. and dawn each night. The waning moon, rising a little later each night through the week, reaches last quarter on Thursday, the day of the equinox. Early on the 23 – at roughly 2 a.m. for mid-northern latitudes – the quarter moon rises in Orion, surrounded by the bright stars and star clusters of Taurus, Gemini, and Auriga. If I was going to lose some sleep for the stars this week, I would pick that night to do it. Thanks for stopping by, and keep looking up. GEJ