Observing - Basics of Astronomy 1

Star Charts and Setting Circles

A star chart is like a road map; it indicates location and manners of measurement.

Memphis Tennessee is at 35 degrees North Latitude - closer (11.5 degrees) to the Tropic of Cancer (23.5 degrees North) than to the Artic Circle (at 68.5 degrees North) 33.5 degrees north than Memphis.

Milwaukee Wisconsin is located at 46 degrees N 12.5 degrees further North than is Memphis - and only 22.5 degrees from the Artic Circle.

Memphis has long hot summers and short cool winters. Milwaukee has short warm summers and longer colder winters.

Lattitude..... is position relative to the poles and the equator. Northern and Southern hemispheres have Artic, Temperate, Tropical, and Equatorial zones.

Declination Setting Circle on an Equatorial Telescope Mount

Declination - DEC

For every Latitude there is a corresponding Declination.  If I am standing in Memphis at 35 degrees Latitude, every star that is at 35 degrees Declination will pass directly over my head at its Zenith where it crosses an imaginary North/South line called the Meridian dividing the eastern half of the sky from the western half. 

Longitude...   ...defines the location of imaginary  lines that run from north to south poles. There are 24 hours in a solar day (a 360 degree rotation of earth relative to the sun). One hour = 15 degrees - the typical width of a “time zone”.

Memphis is 90 degrees West Longitude half way between the Prime Meridian - 6 hours east which runs through Great Britain, and the International Dateline - 6 hours west near Japan.

People in Memphis eat lunch about the same time  Japan is waking up, and also about the same time as Great Britain is having dinner.

Right Ascension Setting Circle on an Equatorial Telescope Mount

Right Ascension  - R A ...

...identifies  north - south lines in space, whereas Longitude identifies north-south lines on earth.

The earth rotates - relative to the stars - once every 23 hours and 56 minutes: a SIDERIAL or  “STAR “ DAY.

Each day, earth also moves 1,584,00 miles along it’s 360 degree, 365 day orbit of the sun - a  bit less than one degree per day. Earth rotates 360 degrees  in one SIDERIAL DAY relative to the stars, but it will take four more minutes (One degree) to complete a Solar Day. 23 hours and 56 minutes + 4 minutes = ome 24 hour day.

SInce a Siderial Day is 4 minutes shorter than a Solar or “calendar” Day, the whole night sky shifts about 4 minutes toward the west each day. Therefore the night sky appears to make one revolution per year.

While Longitude combined with Latitude determines  points on earth, Right Ascension combined with Declination determines the positions of seemingly fixed objects in deep space. Solar System planets and satellites have Right Ascension and Declination coordinates which change constantly as they orbit the sun.

Telescope mounts come in two basic flavors: 

Polar aligned telescope mounts have one axis parallel to that of Earth which rotates west to east and another which pivots north south. These mounts are exclusively for astronomy and allow you to track an object by simply pivoting at the same rate the earth rotates on the north south axis. This is done manually, or by analog or computer timing devices. 

Altitude (up and down) / Azimuth (left and right) mounts (ALTAZIMUTH or ALTAZ) have a vertical and a horizontal axis like a camera tripod. These mounts were at one time mostly for terrestrial use but have long been used for astronomy since the first telescope pointed skyward. The main disadvantage in astronomical usage is that one must pivot the telescope on both axies to track a target.  Fortunately inexpensive computerized systems can make the telescope track and even locate objects regardless of mount type.

So how do you begin finding your way? 

One needs a road map on earth in order to find specific geographical places.   Likewise you need a map of the heavens -as seen from earth - to find objects in space. 

1) Visit http://www.skyandtelescope.com and register on their web site. This will allow you access to a a Star Chart which will identify objects in the sky - as seen from your area - at a specific time on the current date.  This program provides the basic object identification and locating information about the night sky and how to find your way. You can print simple Star Charts from this program that will allow you identify a variety of interesting “naked eye” objects. A good binocular will help!

2) Visit http://www.astronomy.com This site will also allow you access to a a Star Chart which will identify objects in the sky - as seen from your area - at a specific time on the current date.  There may be a charge.

3) Use you favorite search engine and enter the following search terms:  Star Chart Astronomy Software You’ll find that there are many to choose from. You will not master any of these overnight; you will however be able to go outside in an area away from bright lights and begin to understand how these charts work. Once that process is started, it becomes easier and easier. Anyone who has sufficient patience to use a street or road map will quickly understand.

4) Visit an astronomy club. There you will find people who and use know these processes and are able to explain and demonstrate them.  It sometimes feels awkward being around more knowledgeable people, so be patient and participate. You will find people like myself who thrive on helping people get off to a good start.