THE SOUTHWORTH PLANETARIUM
207-780-4249   www.usm.maine.edu/planet
70 Falmouth Street   Portland, Maine 04103
43.6667° N                   70.2667° W 
Altitude:  10 feet below sea level
Founded January 1970
Julian date: 2458748.5
2019-2020:  XV
              "Heavens above!"

THE DAILY ASTRONOMER
Monday, September 23, 2019
Another Astronomical Autumn

started at 3:50 a.m. today.  Now, we're passing through the very first hours of autumn 2019.  Today's DA celebrates autumn's arrival by clarifying some issues pertaining to fall.    Regard this article as a pleasantly un-poetic exploration of astronomical autumn.

IT'S NOT EXACTLY "EQUAL NIGHT" TONIGHT
Here, in Portland, ME, the sun rose this morning at 6:29 a.m. and will set at 6:37 p.m.   Not exactly 12 hours of sunlight and twelve hours of darkness.*   Even though the word "equinox" derives from the term for "equal night," the parity between day and night does not occur precisely on the equinox.     The actual "equal night" date depends on latitude.   At 40 degrees north, the date is September 26th; at 45 degrees north, it is about September 25th.    The Sun rises at 6:33 a.m. and sets at 6:31 p.m. on September 25th here in Portland, which is about latitude 43.5.      The lower the latitude, the later into the fall this "equal night" date occurs.   Around the Tropic of Cancer (23.5 degrees), the equal night date is September 28th.  At 15 degrees, it occurs around September 30th.  At 5 degrees north, October 17th.  Note: The day length is just about 12 hours along the equator each day of the year.    These days of equal day and night are known as "equilux" days.   

The lack of equality is based principally on two factors.   First, if the Sun were a point in space as opposed to a disk, the Sunrise and Sunset times would be almost 12 hours apart on the equinox.  However, the Sun is a disk about half a degree in angular diameter. Sunrise is considered the moment the Sun's upper limb first protrudes above the horizon.  This limb will reach the horizon before the Sun's center does.    Secondly, we have to contend with atmospheric refraction: the way by which Earth's air "shifts" the position of celestial objects. Earth's air shifts the apparent position of celestial objects slightly.  The greatest distortion (about 1/2 a degree) occurs along the horizon, where the atmosphere is thickest between the observer and the sky.   At mid latitudes, this distortion will add about 7 - 8 minutes of sunlight to the sunlight duration around the equinoxes.   

SUN IS STILL VISIBLE AT THE NORTH POLE
Were we to visit the North Pole at this very moment, we would still see the Sun.  Granted, it would be low on the horizon.  If Earth had no atmosphere, the Sun would set at the North Pole on the autumnal equinox.  However, the same atmospheric refraction we mentioned in the earlier section also causes the Sun's image to be projected above the horizon at the North Pole on the equinox, when the actual Sun is sinking below it.    In about two days from now, the Sun will finally appear to dip below the horizon at the North Pole.     

refraction-sunrise-sunset.jpg

NOT THE SAME DAY EVERY YEAR?
One might wonder why the autumnal equinox isn't the same date each year.  Well, it would if Earth's orbital period were precisely 365 days.  Alas, it isn't!    Earth's orbital period is about 365.2422 days.    This discrepancy makes calendar reckoning a tricky business.    It also causes seasonal transition dates to occur at slightly different times each year.      Let's regard the following autumnal equinox dates (Eastern time)

2019:    3:50 a.m.   23 September  

2018:    9:54 p.m.   22 September

2017:    4:02 p.m.   22 September

2016:    10:21 a.m.  22 September

Notice that the autumnal equinox occurs about 6 hours later each year.   Now, based on this list, what would you expect the autumnal equinox date to be in 2020?    Let's look:

2020:  9:31 a.m.   22 September

What happened?!     Leap year happened.     In order to take the quarter day difference between the calendar years into account, an extra day is inserted into a calendar year every four years.  We will experience 366 days between the successive autumnal equinoxes.   If no leap day were included in the 2020 calendar year, the autumnal equinox would occur at 9:31 a.m.  September 23.  About six hours later than the 2019 autumnal equinox at 3:50 a.m. September 23.   

Welcome to another astronomical autumn in the northern hemisphere.    


*Yes, we all know that the darkness doesn't start at sunset.  We're neglecting the twilight periods (Civil, Nautical, and Astronomical.)  


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