Beginner's Observing Guide

Introduction

Perhaps you’ve seen those points of light (some steady; some flashing) in the late evening or early morning hours with your naked eye while looking at the night sky. Some appear to be moving relatively fast, while others are relatively slow. If you observe the night sky with binoculars or a telescope then you’ve probably seen the same thing quickly crossing your field of view.

This page will quickly get you started on predicting when you can see these objects with your naked eye (or binoculars) and be fairly certain what satellite you are viewing.

Why you can see earth orbiting satellites

Satellites are visible when the sky is dark and the satellite is able to reflect sunlight back to the observer. These conditions generally occur up to about 45 minutes before sunrise (before the sky becomes too light) and 45 minutes after sunset. Most satellites are not visible all night long. Sooner or later the earth’s shadow prevents sunlight from illuminating the satellite. This is particularly true for the lower orbiting satellites which enter the earth’s shadow earlier than higher orbiting satellites.

Even though the satellite is in sunlight, to the observer it may only be partially illuminated depending where in the sky it is located with respect to the sun and the observer. Just as the moon goes through lighted phases, so do satellites but in a much faster manner as they transit the sky during a pass. Because of the distance involved and the small size of the satellite you will not be able to resolve the phase of the illumination, only the intensity of the reflected light.

Of course some satellites are bigger than others and have much better reflective surfaces. The bigger and more reflective the satellite happens to be the more light can be reflected and the brighter the satellite will be to the observer.

Regardless how bright the satellite may be, you need clear, and relatively dark skies to observe them. If because of light pollution you can only see the brightest stars and planets, then you will only be able to see the brightest satellites and there aren’t many of those.

Tools

The best resource on the Internet for predicting observations for low earth orbiting satellites is Chris Peat’s Heavens-Above site. Without running your own prediction software, you can predict the brightest (easiest) satellites to observe to the dimmer (more difficult) satellites. The great thing is if the satellite is observable for your period of interest it will be predicted and you won’t waste time trying to predict satellites that are not observable for the period of time in question because they are either too low in the sky or the sky is not dark enough.

Terms

Azimuth-horizontal distance in angular degrees measured in a clockwise direction from north Azimuth is measured in degrees, corresponding to the points on a compass heading on the local horizon. To accurately locate an object, the observer must become familiar with the location directions on his local horizon in terms of compass heading, where both 0 and 360 degrees correspond to true North; 90 degrees corresponds to true East; 180 degrees corresponds to true South; and 270 degrees corresponds to true West. Azimuth angles are "true" (i.e., geographic) headings, not magnetic headings. Northern Observers For observers in the northern hemisphere, the star Polaris is currently less than 1 degree misaligned from true North and is therefore a useful guide for locating the four cardinal points of the compass heading on the local horizon. Knowing where Polaris, the North Star is located, you can then estimate the four geographical cardinal points; North, East, South and West in determining the azimuth angle given in your satellite prediction program. One method to locate the magnitude +2 star Polaris is to locate the Big Dipper and use the two stars that make up the end of the Dipper's cup. These two stars point toward Polaris about 5 times the distance between the two pointing stars. Polaris will be at an altitude equivalent your latitude. If you have Java capabilities on your computer, you can click on the link for the constellation Ursa Major where the Big Dipper resides. This will allow you to view the relationship between the Big Dipper and the Little Dipper (where Polaris is located). Southern Observers Observers in the southern latitudes do not have an easily visible pole star for a reference point. If you have Java capabilities on your computer, you can click on the link for the constellation Crux, a small constellation known as the Southern Cross, a diamond-shaped constellation. Locating the Southern Cross will provide an approximate reference for the South Pole. The Southern Cross will vary up to approximately 30 degrees from true South depending upon the time and the season. A more accurate determination would be to locate the Southern Cross and the +1 magnitude star Achernar. Halfway between the two objects or 3 1/2 Southern Cross lengths toward Achernar would be the South Pole. See this visualization for a better understanding. Another possible suggestion would be to use a magnetic compass to obtain the magnetic (not true) azimuth heading. Depending upon your location on the globe there will a variation between true and magnetic headings called magnetic declination. The magnetic deviation for a given location can be found here. You apply (add, if "west" or subtract, if "east") this variation to the magnetic compass heading to obtain the true azimuth heading. Don't worry if you can't determine the variation at your location. The relatively small error in heading won't significantly affect the naked eye observer because of your wide field of view. The Heavens-Above site provides a sky chart for your location and if you use it just prior to your observation, you will be able to identify the major constellations on your horizon to help locate the azimuth heading needed to find your satellite. There is another method to determine your northern or southern reference point during the day before you begin your night observations. One can measure when the shadow length of a vertical stick is a certain length before local Noon and again when it's the same length after Noon. The true north/south direction is a line bisecting the angle formed by the two lines. To repeat - If you are using your naked eye to make observations you only have to approximate your required azimuth heading as you will have a wide field of view. Altitude-angular distance measured in degrees above the local horizon An altitude of 30 degrees would mean that the object is located 30 degrees above the local horizon. (Note, 10 degrees can be approximated by the width of one's fist held at arms length, so an object at 30 degrees altitude would appear to be approximately three fist widths above the horizon.) An object having an altitude of 0 degrees would be directly on the observer's local horizon. An object having an altitude of 90 degrees would be on the observer's zenith (directly overhead). Once you determine the azimuth (heading) you also need the ability to estimate the height (altitude) above your local horizon in degrees in order to locate the satellite. The Heavens-Above site will provide you the altitude value in addition to the azimuth value. Surprisingly, altitude generally is more difficult to determine than azimuth. Beginners usually underestimate an objects predicted altitude the higher the object is predicted to be located in the sky except for objects near the local zenith (90 degrees or directly overhead). Coordinates (Your location on the globe) The Heavens-Above introductory page will provide the database to obtain your geographical coordinates in terms of latitude and longitude. If you already know these values you can enter them manually. Note the warning to enter the western longitudes correctly. Determining your coordinates within 50 km or ½ of a degree will be adequate to locate your satellite. If you want to successfully predict Iridium flares you will have to determine your location within 1 kilometer (0.01 degree) as the observed flare is only tens of kilometers wide at the earth’s surface. Once you find your proper coordinates be sure to save the page in your "bookmark" or "favorite page" feature on your web browser so you don’t have to repeat this process for future predictions. Time In order to successfully observe predictions you need an accurate time source to accurately set your timepiece used in observing. The timepiece will be used to know when to look for your satellite. The Heavens-above site does provide a time stamp resource. Its accuracy is dependent upon how fast your computer receives the stamp once you request it. Compared to a WWV radio time signal the delay on the US east coast appears to be only a couple of seconds. This is more than adequate to spot a satellite based on a time prediction. Your Internet providers' response time will probably be different.

Having delayed your start with all of the above information, please now go to the



Be sure to share your first observation results with other observers on SeeSat-L .

Supplemental Information

Below is a list of some satellites a beginning observer may find interesting to observe because of their visual characteristics. The Heavens-Above site allows you to search for these satellites by entering either their common NAME, US STRATEGIC COMMAND or INTERNATIONAL LAUNCH designators.

USStrat International Command launch desig. NAME COMMENTS ----- ------ ----------- ------------------------------------ 20580 90037A HST generally +3 magnitude, may flare 21147 91017A Lacrosse 2 +3 magnitude, reddish 21701 91063B UARS +3 magnitude, reddish brown 22823 93061A Spot 3 tumbling, may provide (-)mag. flashes 24836 97030A Iridium 914 tumbling, may provide (-)mag. flashes 24842 97030G Iridium 911 tumbling, may provide (-)mag. flashes 24871 97034C Iridium 920 tumbling, may provide (-)mag. flashes 25017 97064A Lacrosse 3 +3 magnitude, reddish 25063 97074A TRMM +2.5 magnitude 25105 97082B Iridium 24 tumbling, may provide (-)mag. flashes 25320 98026B Iridium 71 tumbling, may provide (-)mag. flashes 25544 98067A ISS very bright (zero mag.) 25861 99039B Okean-0r +2.5 mag. flasher 26040 99072A Cosmos2367 +2.5 mag. 26070 00006B Cosmos2369r +2.5 mag. 26354 00023A Cosmos2370 +2.5 mag. 26473 00047A Lacrosse 4 +3 mag., reddish 26474 00047B Lacrosse 4r +3 mag. 28646 05016A Lacrosse 5 +2 mag 28647 05016B Lacrosse 5r +3 mag.

Related Links

Celestial Coordinates

Visual Satellite Observer's Magnitude system of brightness

Visual Satellite Observer's some Bright Satellites

Visual Satellite Observer's Catch a Flaring/Glinting Iridium satellite

SeeSat-L FAQ

Astronomical Society of South Australia