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: 2458644.5
"The coziest home is an untroubled mind."
-Norman Bates
THE DAILY ASTRONOMER
Monday, June 10, 2019
Opposition Research
Imagine at this very moment that you are observing the solar system from
what we would inaccurately describe as an "aerial viewpoint." More
precisely, imagine you are observing the Sun and its attendant planets from
a vantage point perpendicular to the plane of motion. Consequently, you'd
see the incandescently bright Sun in the center of a series of concentric
planetary orbits. Just for fun, allow each planet to leave a lighted
arc in its wake so that it traces out its own orbit as a luminescent
ellipse. If you look closely, you will see the minuscule aquamarine
Earth gliding between the brilliant Sun and the considerably larger
Jupiter. It's happening right about at this moment. We call this
moment "opposition," when a planet farther from the Sun than Earth appears
opposite the Sun in our sky. This configuration can only occur when our
home planet passes between the other planet and the Sun. Jupiter is at
opposition today!
That is all very well and, to some, perhaps, intensely interesting.
However, one must wonder why such information should be important to
astronomers and what it can tell us about Jupiter's visibility.
*BRIGHTNESS*
First, a planet at opposition will be at its brightest of the year because
a superior planet reaches its point of least distance from Earth around
this time. In the somewhat exaggerated -and certainly not to scale-
graphic below, one can see how much closer a superior planet will be to
Earth at opposition than at other points such as quadrature or conjunction.
When determining a planet's brightness, distance matters. A planet's
brightness relates to its distance from the Sun and its distance from
Earth, both of which change continuously. * We know that when a superior
planet is approaching opposition, it will tend to grow brighter.
[image: opposition.jpg]
*MOTION*
Generally, planets appear to move from west to east due to their
revolutionary motion around the Sun. However, at times, a planet will
appear to become stationary before moving east to west as it moves along
"retrograde motion." Superior planets appear to move backward when the
faster planet Earth approaches and then passes them along its orbit.
This approach and passage can only occur during the time period immediately
preceding, during, and immediately following a planetary opposition.
Jupiter started its retrograde loop on April 10th. It reaches opposition
today, June 10th, and will resume prograde motion on August 11th. One can
determine when a planet will be following a retrograde loop by knowing when
it is due to be in opposition.
*FUTURE POSITION*
Jupiter's average synodic period is about 399 days. This means that
successive oppositions of Jupiter will occur about every 399 days. The
next opposition of Jupiter occurs on July 14, 2020, which is 400 days from
today. The synodic period varies slightly because both Earth and Jupiter
are always at varying distances from the Sun and therefore do not revolve
at constant speeds. We know that Jupiter will be a brilliant summer
planet this year as well as the next one and the one following, even if it
does reach opposition later in the year in both 2020 and 2021.
Every piece of information pertaining to a planet's orbit is useful to
astronomers who want to describe it motion and predict future
positions. Knowing when a planet's opposition occurs will help us know
when it will be at its brightest, when it will be moving backward and when
it will be at opposition in the future.
*The other factors determining a planet's brightness are its albedo (the
ratio of reflected to received sunlight) and size. For Saturn, the ring
aspect, the ring's angle relative to Earth and the Sun, contributes to the
planet's brightness.
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