THE SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
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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.
[image: 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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