Constellations and Asterisms
Constellations When ancient people looked at the sky they saw patterns and gave names to apparent groups of stars. Constellation patterns have been found on tablets over 4000 years old in the Euphrates River Valley. A group of stars may appear to be connected, however they are usually very far apart. 2 stars can appear to be close together in the sky, but one might be only 10 light years from Earth, while the other is 1000. 2
Example: Orion's Belt The 3 stars that make up Orion's belt are Alnitak, Alnilam and Mintaka appear to be very close together, but Mintaka is 100 light years farther away than Alnitak (which is the closest), and Alnilam is 200 light years farther away. 3
Asterisms vs. Constellations An asterism is a group of recognizable stars that is not recognized as an official constellation. An area in the sky defines a constellation. A constellation will often contain an asterism within its area. This picture shows how the sky is divided up into areas based on the constellations. 4
Ursa Major & The Big Dipper The Big Dipper (an asterism) is a part of Ursa Major (a constellation). Ursa Major means "Big Bear". That bear looks pretty messed up to me. What's with the tail?! 5
The Ursa Major constellation (along with all constellations) is an area of the sky (shown in the right picture) not a group of specific stars. 6
Asterisms are like a group of cities and highways on a map. Constellations are like the province that the cities are in. 7
Circumpolar Stars A circumpolar star is a star that, as viewed from a given latitude (distance from the equator) never sets (disappears below the horizon) due to its proximity to one of the poles. Circumpolar stars are visible (from said location) for the entire night, every night of the year (and throughout the day too, although they're overwhelmed by the Sun's glare). 8
All stars appear to move in circles in the sky because of the Earth's rotation, except for Polaris which is at the celestial north pole. Polaris 9
For this particular location, all stars inside the circle shown are circumpolar, while all stars outside of it are not (more or less, thanks to the bumpy horizon). 10
Seasonal Patterns Although some of the circumpolar constellations (such as Ursa Major and Ursa Minor) are visible all year round, others (like Cygnus and Orion) are only visible at certain times of the year. 11
During the summer night, people in the northern hemisphere see Cygnus, but cannot see Orion since it is behind the Sun. In the winter, people in the northern hemisphere do see Orion since the northern latitudes point towards Orion and the Sun sits between Earth and Cygnus. 12
The stars themselves never "go away", it's just that half of the sky is too bright (thanks to the Sun) to see the stars. 13
If we wait 6 months, we'll be on the other side of the Sun, and the opposite side of the sky will be visible to us. 14
The idea is why we can sometimes see the Moon during the day. The Moon (and a load of stars) are always there, it's just that normally the Sun is so bright that we can't see them. 15
Sky Maps Sky maps are used by both professional an amateur astronomers. Since the Earth is constantly moving, the night sky seems to constantly change. The sky changes over a single night because of rotation, and over a year because of revolution. The position of constellations and asterisms depend on where you're looking from, what day of the year, and what time of the day it is. 16
Sky Map - Kitchener, Ontario Vernal Equinox (March 20 th, Midnight) 17
Sky Map - Kitchener, Ontario Autumnal Equinox (Sept. 23 rd, Midnight) 18
Important Notes About Sky Maps... You can find and make maps for any date / time by searching on the internet. Objects in the middle of the map will be directly overhead and those near the outer part of the circle will be found on the horizon. Although it looks like East and West are backwards on the map, it's just because regular maps are placed below you, while sky maps are made to be held above you. Think about this one... it'll make sense! 19
Zodiac Constellations The Zodiac constellations from astrology (which is NOT a science) are the constellations that fall along the apparent path of the Sun throughout the year. 20
The Sun spends about 1 month in each of the 12 constellations of the Zodiac. 21