If this series of articles
has motivated you to get
outside and look up then
you have probably noticed
by now that each night most
stars rise in the east and set
in the west, just like the Sun
does during the day, while
the stars in our northern sky
never seem to rise or set but
appear to rotate in a counterclockwise
manner around
the North Star, Polaris.
This of course is all an illusion,
the stars aren’t really
rising and setting or tracing
out circles in our sky, it is
just an optical effect created
by our planet rotating upon
its axis.
Over the course of weeks
and months you will have
noticed that the familiar star
patterns of one season eventually
gives way to another
as time goes by.
This is due to Earth’s orbital
motion around the Sun.
To be fair, the stars are
indeed moving but they are
so far away from us that their
motions are barely discernable
to us.
The planets however are
much closer and you may
have noticed that they tend
to wander around quite a
bit against the background
stars.
In fact, the very word
“planet” means “wanderer.”
Let’s take Mars as an
example. Say that you go
out one night at around
9pm (during a time of the
year when Mars is indeed
up in the evening hours) and
you notice that Mars is very
close to a particular star
but then you go out again
the next night at the same
time and something strange
seems to have happened:
Mars is no longer near that
star!
What’s up with that? Well,
Mars is in motion just like
everything else is in the
universe.
Over the past 24 hours
it has journeyed 2 million
kilometers around the Sun
while the Earth has been a
bit speedier and has covered
2.5 million kilometers in its
orbit. From our perspective
here on Earth all the planets
appear to roam around the
sky within the “ecliptic”, that
path across the sky through
which the Sun and Moon
appear to move over the
course of a year.
For several nights running
an outer planet (like
Mars, Jupiter, or Saturn)
may appear to be heading
in one direction but then a
few nights later it seems to
reverse course and go in
the opposite direction. This
about-face in direction is
known as “retrograde motion.”
Because we are completing
a smaller journey around
the Sun than does an outer
planet we overtaken them
along the way and pass them
by. It’s a bit like overtaking a
car on a racetrack and passing
it by.
That car isn’t really travelling
backwards; it just looks
that way as you pass it up.
Same thing with the planets
as we all engage in our NASCAR
race around the Sun.
If you are one of those
people who are out and
about before sunrise then
you may have noticed this
planet wandering-action taking
place over the past few
weeks in our eastern skies
right around dawn.
Mars, Jupiter, and Venus
have been doing a little skipto-
my-Lou here of late as
they have been converging
and then passing each other
by in a lovely little dance during
the wee hours just before
the Sun rises to put a damper
on the show.
If you want the technical
name for this kind of planet
grouping in our sky then you
can amaze and impress your
friends with something like:
“Hey, have you seen that
amazing conjunction in the
east just before dawn?”
In addition to Venus,
Mars, and Jupiter we also
have the elusive planet Mercury
entering the scene
briefly during mid-October
but it drops out over the
course of a few mornings
after the 17th.
To keep track of who’s
who you can make use of
various astronomy apps or
you can go to the In-the-Sky
website to use their planetarium
feature in order to
make sense of the planetary
dance card: https://in-thesky.
org/index.php
Be sure and take a good
look on the morning of Oct.
23 to see the three primary
players forming a triangle
tight enough to where all
three will fit within a binocular
field of view.
Now, you don’t need binoculars
to see them but if
you do have them break
them out to see if you can’t
spot some of Jupiter’s four
largest moons: Io, Europa,
Ganymede, and Callisto.
After the 23rd the triangle
breaks up and it will form
again on the 28th but with a
different configuration.
Planet watching can be
a lot of fun, especially when
you know who is who, what
is what, and why they are all
acting the way they are.
Get out, look up, and enjoy
• • •
Darrell Heath works at the University of Arkansas at Little Rock and is a producer and host for the UALR Television show
“The Night Sky.”
