CONSTELLATIONS



To most of us, constellations are the familiar patterns of stars that map the sky, such as the Big and Little Dippers. To astronomers, the constellations are regions of the sky, much like states on a map, known by Latin names such as Ursa Major and Ursa Minor. There are 88 officially defined constellations although in traditional cultures they may be named differently. Remarkably, the Big Bear is almost always the Big Bear (Ursa Major) even though bears are not known to have the long tail this pattern seems to show. What we call the Big Dipper is part of the larger constellation of Ursa Major.



Most constellations appear to cross the sky east to west each night, like the Sun and Moon, because the Earth rotates west to east beneath them.


SEASONAL CONSTELLATIONS

As the Earth travels counterclockwise around the Sun each year, night looks out towards different parts of the Universe. At different seasons, different constellations cross the sky. Since there are 365 days in a year, and 360 degrees in a circle, the stars shift about one degree westward each night.



CIRCUMPOLAR STARS

Stars near the poles do not rise and set, but appear to circle the poles.

In the Northern hemisphere circumpolar stars circle near Polaris, the North Star at the tip of the Little Bear's tail. Click here to see how to find Polaris.

These stars are found in constellations such as Ursa Major, Ursa Minor, and Cassiopeia.


The northerm Gemini telescope on Mauna Kea, Hawaii, with north circumpolar star trails caused by the rotation of the earth.
Credit: Gemini Observatory/NOAO/AURA/NSF

In the Southern hemisphere circumpolar stars circle over the South Pole, such as in the constellation Carina. One particular star in the constellation is Eta Carina, a strange star expected to become a supernova, if it hasn't already. Read more about it here on Astronomy Picture of the Day. Nearby galaxies, the Large and Small Magellanic Clouds, look like thumb-prints on the southern sky.


Circumpolar star trails around the south celestial pole, as seen from Australia.
Credit: Tony Dutton

While the North Star, Polaris, cannot be seen from the southern hemisphere, the Large and Small Magellanic Clouds that are visible in this image and the unstable star Eta Carina cannot be seen from the northern hemisphere.


Eta Carina
Credit: Greg Bond



ECLIPTIC

When we look at the ecliptic, which runs through the constellations of the Zodiac. Aries (Ram), Taurus the Bull, Gemini the Twins, Cancer the Crab, Leo the Lion, Virgo the Virgin, Libra the Scale, Scorpio (Scorpion), Sagittarius (Archer), Capricorn (Goat), Aquarius the Water Carrier, Pisces the Fish, and Ophicus (Serpent Handler), we are actually looking out across the plane of the Solar System, in which the orbits of the planets around the Sun. Consequently it is where we find the Sun, Moon, and planets in the sky.

Play this movie which shows the Sun moving around the celestial sphere over the course of the year. It traces that motion out as the ecliptic, and zooms in to show where the Sun would appear on the solstices and the equinoxes. The movie shows the "full" celestial sphere model, where the Sun is just one more object on the surface of the celestial sphere. (Credit: Robert Knop, Vanderbilt University; Earth texture by Rick Kohrs.)

The Sun's apparent motion is also shown in this movie that shows how the moving Earth changes our perception of where the Sun seems to be. (Credit: Robert Knop , Vanderbilt University; Earth texture by Rick Kohrs.)

The Zodiac (meaning little animals) was invented by the Babylonians, in Iraq, about 5th century BC. At that time, if someone was said to be born under a certain sign, it meant the Sun was in front of that constellation the day they were born. Since the constellation cannot be seen in the daylight, the constellation nearest the Sun as sunset and the constellation nearest the Sun at sunrise are noted, and the missing constellation between them is the sign in which the Sun is found

Due to a wobble in the Earth's axis the direction of the seasons shifts, making a full cycle every 26,000 years. The signs of the Zodiac are now off by about a month. So folks are now born when the Sun is in front of the sign attributed to the previous month. Someone born in August is likely born when the Sun is in front of Leo rather than Virgo. The easiest way to see one's Zodiac constellation is to see what constellation is overhead at midnight, 6 months after one's birthday.



CELESTIAL EQUATOR

The celestial equator is the plane made by the Earth's equator if it were extended out into space. The celestial equator tilts 23.5 degrees from the ecliptic, due to the Earth's axial tilt.

The Earth's tilt and celestial equator remain the same throughout the year as it Earth travels around the Sun during the year. Sunshine changes its illumination of the northern and southern hemispheres, causing the seasons.

When the Earth is on one side of the Sun tilting towards the Sun, it is Summer in the Northern Hemisphere and Winter in the Southern Hemisphere. Six months later, when the Earth is on the other side of the Sun, the tilt is away from the Sun, causing fewer rays from the Sun to strike the Northern Hemisphere causing Winter, while there is Summer in the Southern Hemisphere.



MESSIER AND NGC NUMBERS

At age 14 Charles Messier was inspired to become an astronomer when he saw a 6-tailed comet in 1744 and later an annular solar eclipse in 1748. Charles Messier (1730 - 1817) a French clerk made a list of 110 bothersome smudges not to be confused with comets which were all the rage.


To read more about Messier and his objects, see his entry in Wikipedia, and a description of the Messier object list. An interactive starchart of Messier objects is available at http://www.ngc891.com.

These objects were given M numbers, such as: