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.

Road maps indicate relative locations, and routes of travel.  Space maps indicate object locations at given points in time.  Modern conveniences like GPS and electronic mapping systems tend to make us lazy. If you make an effort to learn the basics, you can get by without the gizmos much better than you might imagine.


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Star charts have special counterparts of Latitude and Longitude used on terrestrial maps:

Declination (DEC) and Right Ascension (RA)


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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 of Memphis.

Milwaukee Wisconsin is located at 46 degrees N 12.5 degrees further North than is Memphis - and only 22.5 degrees from the Arctic 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 Earth’s poles and equator. Northern and Southern hemispheres have Artic, Temperate, Tropical, and Equatorial zones.

Declination - DEC

Declination Setting Circle on an Equatorial Telescope Mount

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  - R A ...

Right Ascension Setting Circle on an Equatorial Telescope Mount

...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 = one 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  coordinates which change constantly as they orbit the sun. Declination coordinates remain the same.

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 the telescope east to west at the same rate the earth rotates on the north south axis west to east.  This is done manually, by a simple time clock motor, or computerized 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 axes to track a target.  Fortunately inexpensive computerized systems can make the telescope track and even locate objects regardless of mount type.

Many equatorial telescope mounts have actual setting circles as described here, with some scopes since 1980 having digital readout setting circle technology, and many since 1990 having the ability to aim themselves via computer controlled motors.  Old fashioned setting circles are too cool.

In order for setting circles to work, 1) the polar axis of the telescope must be pointed as close as possible to the NCP (North Celestial Pole) which is very close to the 2nd magnitude star Polaris,  2) the telescope must be aimed at an identifiable star near the much dimmer object you wish to find, and 3) the setting circles must be turned so that their pointers are set at the coordinates for the star you have chosen.  4) Then move the RA and DEC axes so that the pointers move to the desired “celestial address” of the object you wish to see. This information is available in an star catalog. Start with a low power eyepiece that reveals a larger area of sky so that you are less likely to overlook the object.

Burnam’s 3 part series is an ideal “little black book: describing thousands of objects, their character, and their “addresses”.

I own Burnam’s and Nightwatch . Nightwatch is very user friendly and disseminates RA and DEC plus other information for using setting circles and simpler methods. Click here to sample.

If you know your compass points and use brighter objects as visual reference points in combination with one’s hand extended at arm’s length can allow you to “feel” your way toward the “faint fuzzies” for which you are looking.  I recommend holding your hand and fingers up against the sky to compare their sizes to that of the full moon - which is 1/2 degree across.  A number of fingers, a fist, open hand, etc. represents a given degree count that your mind can translated in your mind can very accurately process -  well enough to get get yo on target.  See the One Minute Astronomer


Declination Setting Circle

Right Ascension Setting Circle

Setting circles are a “throwback” to previous generations before digital electronic readouts.

Learning the basics of earlier technologies helps us appreciate new technologies better - and often to get along without them.

Modern “Go -To” telescopes do the positioning for you by virtue of computerized databases.

This Meade Model 826 Newtonian is a classic. Many people prefer vintage models over newer models.  Mastering it can be very rewarding.   The 826 is on seriously beautiful  telescope.


I do not use GPS devices for traveling.  I do not own an I Phone.  It is funny - I have no problem living without these. As a wealthy hobbyist might own and fly an old World War 2 airplane, many of us get great enjoyment in preserving and using the “high technology” of the past.

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 http://www.astronomy.com register on their web sites. This will allow you to access star chart which will identify objects in the sky - as seen from your area - at a specific time on the current date.  Many other relevant resources are available.

2) Visit One Minute Astronomer . The trick to enjoying astronomy is arming yourself with relevant information.  I have introduced individuals to astronomy only to discover that they know more than I do within a year! When I was younger, we just did not have the educational resources we have today.

3) Use you favorite search engine and enter astronomical search terms - like “nebula”, “star cluster”, “creation”, etc. You’ll find that there are many paths into and around this hobby. You’ll find places where the scientific information seduces you into thinking about exotic possibilities in the space time continuum.

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.



Pale Blue Dot

Observing Basics 1

Observing Basics 2

Seeing the Cosmos

 Telescope Crash Course

Astro  Video LINKS

 Telescope Basics

Astronomy Optics and Systems

Oil Capital Observing Center

Map and Directions



Weather and Moon Phases

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Astronomy Optics / System