Daniel Pope

Capricornus (CAP-rih-CORN-us), the Sea Goat, is on the Ecliptic, the plane of our Solar System, and is traditionally listed as the tenth member of the Zodiac. The stars that form Capricornus are rather dim and difficult to observe. Since it is on the Ecliptic, periodically one of our planets will pass through this elusive constellation.  We are fortunate that is the case right now in the form of mighty Jupiter.  As soon as it is dark we can see brilliant Jupiter high in the southeast.  I agree with those you say Capricornus looks more like an old fashion sail than a Sea Goat.  Your closed fist at arm’s length will just about fill the interior of Capricornus.

Even though to the naked eye there is not much to see in this part of the sky, astronomers are taking a closer look because exo-solar planets have been detected around stars in Capricornus. Capricornus’ name will be forever preserved on our globes, maps, and atlases as the Tropic of Capricorn.  About 2,000 years ago our Sun was in Capricornus as it reached its most southern point in our sky on the first day of winter – our Winter Solstice.  The Tropic of Capricorn refers to an imaginary line drawn around our Earth at latitude 23.5 degrees south, the most southerly latitude at which our Sun is ever directly overhead.  In the present era our Sun is in the next constellation to the west, Sagittarius, on the first day of winter.  There has been no talk of changing the name of the 23.5-degree line to the Tropic of Sagittarius.  This apparent movement of our Winter Solstice is due to a very subtle motion of our Earth, called precession.  In another 24,000 years our Winter Solstice will be back in Capricornus.

When I gaze at Capricornus, the amazing story of the discovery of our 8th planet, Neptune, among these stars comes to mind.  Neptune is not visible to the naked eye but it is now almost exactly where it was when it was discovered on September 23,1846 by Johann Galle – 163 years ago.  It takes Neptune 164.79 years to complete one orbit of our Sun.  On July 12, 2011 Neptune will have completed one orbit of our Sun since its discovery. 

William Herschel was using a telescope in 1781 to create a star map when he accidentally discovered our 7th planet, Uranus.  Soon afterward other astronomers started creating tables to predict Uranus’ positions in the future.  By 1821 it was clear that the predictions were not matching the observations – something was wrong.  John Adams and Urbain Le Verrier began independently, and unaware of each other’s work, to calculate where an 8th planet might be that would cause the deviations in Uranus’ orbit.  Le Verrier sent his predictions to Johann Galle and they arrived on September 23, 1846.  Galle aimed his telescope at the predicted spot that very same evening and discovered Neptune within one degree of where Le Verrier calculated it would be.  Newton’s theory of gravity and mathematics were used to predict the existence and position of a new planet.  (Mathematics is a powerful tool!) Neptune was just above the upper leftmost star in Capricornus – Deneb Algiedi, the delta star in Capricornus.

When I don’t have Jupiter or another planet to help me find Capricornus, I use the only reasonably bright star in this region, Fomalhaut (FO-mal-ought).  Fomalhaut is about a hand span at arm’s length to the lower left of Jupiter.  In 2008 the Hubble Space Telescope took the first visible-light picture of a planet orbiting another star and that star is Fomalhaut, just 25 light years away.

The myths about the origin of Capricornus are somewhat confusing and in places contradictory.  One myth identifies Capricornus as the lusty god Pan.  Pan and other gods on his team were taking a break from battles by having a picnic by the Nile River.  The monster Typhon, a member of the other team, made a surprise appearance.  Pan’s group changed themselves into various animals and ran away.  Pan ‘panicked’ – some say the origin of the word – and jumped into the Nile as he was trying to decide what animal to become.  As a result of his indecision (he was thinking goat) his bottom half in the river became a fish tail and his top half became a goat.  This is the origin of the ‘Sea Goat’ that is now Capricornus. 

In other myths this goat is identified as Amalthea, the goat that suckled Zeus when he was an infant.  It was also claimed that the Sun ‘took nourishment’ from the goat before beginning its climb back northward after the winter solstice.

Clear skies.