THE USM 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:  2458654.5
                "Don't fear to be eccentric in opinion, for every opinion
now accepted was once eccentric."
                                      -Bertrand Russell

THE DAILY ASTRONOMER
Thursday, June 20, 2019
Solstice Pandora

The last day of astronomical spring!
At 11:54 a.m. EDT June 21, 2019, astronomical summer will begin.    Prior
to summer's arrival. we wanted to address some questions people have asked
about solstices in general and the summer solstice in particular.      For
these questions, we need only seek out the parchment papers contained
within Pandora's Jar.


*"Is it true that we're actually farthest from the Sun on the summer
solstice?"*
*-K.S,  Naples*
Not exactly.  We're farthest from the Sun (at aphelion) around July 4th.
Earth revolves around the Sun along an elliptical path that is slightly
elongated.    Consequently, as Earth moves around this orbit, it will
attain its point of least distance (perihelion) and that of greatest
distance (aphelion.)   Earth's aphelion distance is 94.5 million miles and
its perihelion distance is 91.5 million miles.    Earth reaches aphelion
about two weeks after the June (summer) solstice.

[image: DMJpxluzTZ6rZ384v5T7_Earth_Orbit-732X5201.jpg]
*Earth's orbital path *is a slightly elongated ellipse.   As Earth moves
around the Sun, it reaches its point of least distance (perihelion) six
months before reaching its point of greatest distance (aphelion.)

*"Will the Sun be overhead on the Equator during each solstice?"*
*-K.F.,   Portland*
The Sun passes overhead on the Equator during each equinox.   On the summer
solstice, the Sun passes overhead at the Tropic of Cancer (23.5 degrees N
latitude.)  The Sun passes overhead at the Tropic of Capricorn (23.5
degrees S) on the December (winter) solstice.     Earth' tilt, or
obliquity, measures 23.5 degrees, hence the latitude values of the two
tropics.    If Earth's obliquity were 43.5 degrees, the Tropic of Cancer
would intersect Portland, ME, and the Sun would be overhead here on the
summer solstice.   We can dream...

*"Do other planets have solstices?"*
*-B.F,  Biddeford*
Yes.    If the planet is titled, it will experience solstices: times when
the Sun reaches its highest and lowest points in both hemispheres.     The
time separating successive solstices will be greater on more distant
planets where the years are longer.   For instance, the next summer
solstice for the Martian northern hemisphere occurs on October 8, 2019.
The following Martian northern hemisphere solstice happens on August 25,
2021.

[image: download.jpeg]
Any planet that is tilted on its axis will
experience solstices, those times when
the Sun attains it lowest and highest points.
The next Martian Northern Hemisphere solstice
occurs on October 8, 2019.     Note that the Martian
tilt, or obliquity, is only slightly larger than Earth's.

*"I know that the Sun's solstice position shifted from Gemini into Taurus
because of Earth's slow wobble.  Does that mean that the dare of the
solstice will change, too?"   -C.C.   Windham*

The Gregorian calendar replaced the Julian calendar so as to prevent the
seasonal date shifting you mentioned.     Such seasonal date displacement
occurred in the Julian calendar because of the discrepancy between the year
length on the calendar and the actual time period Earth requires to
complete an orbit around the Sun.    The Julian calendar did impose a leap
year every four years to create a year length of 365.25 days.   However, as
the year's length is closer to 365.2422 days, the vernal equinox(first day
of spring) slowly shifted relative to March 21st, the equinox date that the
ecclesiastical authorities invariably recognize.   By the time of the
Gregorian reform, the vernal equinox  fell around March 11th.  The reform
eliminated eleven days to reset the equinox on March 21st and then imposed
new rules relative to leap years:
An extra day is inserted into every year equally divisible by four except
for century years, unless those century years are equally divisible by
400.     So, 1600 and 2000 were both leap years.   1700, 1800, and 1900
were not.     This new Gregorian calendar is much more accurate than its
Julian predecessor.   However, owing to a 27 second annual discrepancy, the
calendar will be off by about 1 day every 3260 years.         However, even
as seasonal points migrate along the ecliptic, the dates of the seasons
will remain comfortingly consistent.