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: 2458779.16
2019-2020:  XXXVII
                     "Astronomers, like burglars and jazz musicians,
operate best at night."
                                      -Miles Kington



THE DAILY ASTRONOMER
Wednesday, October 23, 2019
93 Million Miles


It is one of the questions we receive most frequently:  "How far away is
the Sun?" The indoctrination camps we planetarium people attend as children
have conditioned us to provide the immediate response, "93 million
miles." Ironically,
most of the time, this answer is blatantly false, except for two times a
year.   October 6th is one of those times. (Yes, we just missed it)  The
other is April 3rd.   If anybody visits the planetarium on either days and
asks that question, we can proffer the 93 million mile answer without
imperiling our immortal soul or even experiencing any assaults from our
otherwise serene consciences.

One might wonder:  how could this answer not only be false, but generally
false throughout the year?  It all pertains to our planet's motion around
the Sun.  The graphic below shows Earth's orbital path. Notice that the
path is elliptical, not circular.  If Earth's orbit were circular, it would
maintain a constant distance from the Sun throughout the year.   As Earth's
orbit is instead an elongated ellipse, its distance from the Sun changes
continually. Its maximum distance occurs at aphelion around July 4th. At
this time, Earth's distance will  be 94.5 million miles.    Earth reaches
its minimum heliocentric distance at perihelion around January 3.  When at
perihelion, Earth is about 91.5 million miles away.

[image: Earth-orbit-Sun.jpg]
*Earth's elliptical orbit: *  This graphic depicts Earth's motion around
the Sun.  As Earth travels around an ellipse, its distance from the Sun
varies from a maximum of 94.5 million miles (aphelion) to 91.5 million
miles (perihelion)    Earth reaches perihelion around January 3 and
aphelion around July 4th.   Earth is only 93 million miles away from the
Sun during the two dates midway between perihelion and aphelion:   around
October 6 and April 3.   Image: Encyclopedia Britannica

Earth is precisely 93 million miles from the Sun on only two dates, each of
which is midway between the perihelion and aphelion dates:  October 6 and
April 3.  (Due to slight irregularities, these dates can vary by a couple
of days. So, too, will the dates of perihelion and aphelion.)  Since
aphelion occurred in early July and the 93 million mile day happened in
early October, we're actually moving closer to the Sun now.    After
January 3rd, Earth will be moving farther away.

It is said that it is almost impossible to provide a complete answer to any
questions because an immense amount qualifying information goes unsaid.  In
the case of the Sun's distance, much does go unsaid when we mention it.
Of course, physical reality is all the more intriguing and wondrous for all
the opaque regions contained within it.



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