What’s Your Sign?

That question, for those of us of a certain age, is associated most often with the garish 70’s singles scene.  For teachers, it represents part of the struggle to educate students new to astronomy to disabuse the relationship between the location of the stars and planets with one’s personal future outlook.  While a teacher might recoil in horror when an assignment is turned in with the heading Astrology 101, a student’s interest in astrology can be used to teach certain astronomy concepts.  It is somewhat similar using science fiction in the classroom.  After all, part of the purpose of education is to enable students to make conceptual leaps from myth to reality.  And to do that, you have to meet the student on level ground.

Astrology is where many of us first learn of the constellations.  There are 88 constellations that divide the celestial sphere in the same manner states divide a nation.  However, astrology focuses on 12 zodiac constellations that lie in the ecliptic and form a background that the Sun and planets move through.  The ecliptic is a narrow path in the sky that the Sun and planets travel from our perspective on Earth.  As the Solar System formed 4.5 billion years ago, the solar nebula flattened causing the planets and Sun to coalesce in the same disk.  Conceptually, it can be difficult to imagine the Sun and planets moving in the same path in the sky as we see them during different times (the Sun during the day and planets at night).  A total solar eclipse does allow us to visualize this.

EclipseThis is a simulation of the total eclipse to occur on April 8, 2024 in Buffalo.  With the Sun’s light blocked by the Moon, you can see how the planets (in this case, Mercury, Venus, Mars, & Saturn) and Sun move along in the same path in the sky.  And this path is called the ecliptic.  You’ll also note two constellations in the ecliptic, Pisces and Aquarius, which correlate to astrological signs.  This illustrates how the Sun lies in constellations just as the planets do.  We typically do not get to visualize this as the Sun’s brightness does not allow us to see constellations during the day.  This image demonstrates how the zodiac constellations align with the Earth and Sun during the year.

Credit: David Darling

The Sun will be located in the constellation opposite from Earth.  Some caveats here, the month the Sun is located in a constellation will not match your astrological sign.  Also, there actually is another constellation, Ophiuchus, that lies in the ecliptic but the ancient Babylonian astrologers decided to casually toss that one out as they were using a 12 month calendar.  Noting that astrology has not kept up with the precession of the Earth’s axis the last few thousand years will hopefully be a first step in cracking any validity astrology may have with a student.  Note in the eclipse image above the Sun resides in Pisces, but your astrological sign is Taurus if born on April 8th.  Planetarium software such as Starry Night will allow the class to view the changing zodiac throughout time.

Retrograde Motion

Often referred to in astrology, retrograde motion pertains to a “backwards” motion of a planet in the ecliptic.  Before the Copernicus revolution putting the Sun, instead of the Earth, at the center of the Solar System, retrograde motions confounded astronomers.  Lets take a look at an example, Mars during 2016.  The image below tracks Mars motion in the ecliptic from the beginning of 2016 to the end of September, 2016.

MarsretroThe retrograde or backwards motion of Mars occurs from April 17th to June 30th.  What’s happening here?  Earth and Mars are approaching opposition, an event that will take place on May 22, 2016.  At this time, Mars and the Sun are on opposite sides of the Earth.  This means on that date, Mars will rise in the east as the Sun sets in the west.  As this is Mars closest approach to Earth, Mars is at its brightest.  Opposition is the optimal time to observe a planet.  In the case of Mars, impending opposition to Mars also represents launch windows for space agencies to send missions there.  If the launch window is missed, the mission must wait another 26 months until the next opposition.  So how does all this result in retrograde motion?  This is when Earth “passes” Mars like a race car with an inside track passes a car on the outside.  The resulting retrograde effect is visualized below:

Credit: NASA

Point d is when opposition occurs and is the midway point of the retrograde motion.


As the planets all orbit the Sun in the same plane, sometimes they align in the same line of view to form a conjunction of planets.  One such example will occur in the early morning hours of October 28th when Venus, Mars, or Jupiter will all be within a few degrees of each other.  The scene will look like this:

ConjunctionThese conjunctions serve as excellent teaching opportunities as it allows students to locate several planets at once quite easily.  Below is an inner Solar System view of the event.  Jupiter is not in the image but would be aligned right behind Mars if visible in this view:

InnerconjFrom an planetary science perspective, conjunctions really do not have much to offer, but for the rest of us they can provide a really neat night time spectacle.  These events are an excellent way to introduce the planets to those new to astronomy.

Zodiacal Light

I’ve never heard the zodiacal light mentioned in an astrological context but, while we are learning about the zodiac, now is as good as any time to familiarize ourselves with this.  Besides the Sun, planets, and asteroids, the plane of ecliptic is occupied by cosmic dust.  This dust is the remnants of comet tails and asteroid collisions.  Best seen in dark sky locations, the zodiacal light is visible in the east just before sunrise or in the west just after sunset.  This faint glow, which follows the ecliptic in the sky, is usually most observable in the Spring or Fall.  During these seasons, the ecliptic has a steeper pitch relative to the horizon.  Fainter than the Milky Way, most urban dwellers do not get the opportunity to see it.  However, if you find yourself away from the city lights, this is what you can expect to see:

Zodiacal light from VLT in Chile Credit: ESO/Yuri Beletsky


The Sun enters Sagittarius in mid-December and exits during mid-January.  During the Summer months, the classic teapot of Sagittarius lies high in the night sky along with the Milky Way.  In fact, the center of the Milky Way lies in Sagittarius.  What this means is that each December, both the center of the Milky Way and the Sun are in Sagittarius.  You might recall back in 2012, this alignment was, according to some less than reliable sources, going to result in the end of the world.  Needless to say, as an annual event, this would have destroyed the Earth a whole lot sooner than 2012.  That is the power of a liberal arts education, it clears a lot of silly stuff out of the way.

December 21, 2015 – The Sun and Milky Way center both reside in Sagittarius. Daylight was shut off on Starry Night software so both the Sun and constellation can be seen at the same time.

Speaking of which, although Sagittarian Jim Morrison gets it “right” about astrology here, you really should not need a celebrity to tell you what is useful information and what is not.  And the audience is clearly going with whatever Morrison is telling them here.  A liberal arts education provides a basic framework of knowledge to make up your own mind and not be concerned with looking cool or not with the results.

*Image on top of post is the painting of the 12 constellations of the Zodiac on the ceiling of the Grand Central Terminal in New York City.  Infamously, the designer goofed and placed the constellations backwards.  Photo:  Gregory Pijanowski.

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