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: 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. 

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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