Using the comet visibility diagram
The diagram shows the horizontal coordinates of a comet for a user defined altitude of the sun. The default setting is an altitude of -12° which is the limit of nautical dawn, i.e. if the sun is below this limit many stars are already visible. Other appropriate limits are below -15° (almost completely dark) and below -18° (astronomical dawn, completely dark) for fainter comets.
The location of the comet is marked for every day of the selcted range by blue/white filled circles. Every five days, labels show the date and time of the comet's location. The alignment of the comet's tail with respect of the horizon is indicated by the blue/white line emanating from the filled circle.
The gray line with light gray dots shows the location and movement of the comet 30 minutes before and 30 minutes after the time the sun is located at the selected altitude. This allows to estimate how the comet moves in the sky on a certain morning/evening and when it will rise/set. There are light gray dots every 10 minutes.
The computed altitude of the comet takes into account atmospheric refraction! This explaines the sudden jumps/drops of the comet's altitude just below the horizon.
Sometimes a comet will not be visible on a certain day or for a certain location. This is then stated on the diagram. It also sometimes occurs that the sun does not drop below or does not get above the selected altitude. This is usually the case in mid and high latitudes in summer when the sun does not drop below -18°. In the high latitudes in winter, the sun does not rise above -18°. It is stated on the diagram on how many does this is the case. For such dates, comet locations are not computed!
Date: Select the date you want the diagram to be centered on. The number of days which will be shown before and after the selected date can be entered after the ±.
Latitude and longitude: Enter the coordinates of your observing site. Longitude is in degrees East, i.e. negative for degrees West.
UTC offset: The offset of you time zone with respect of UTC. Enter a positive value if you are ahead of UTC (usually east of 0° longitude) and a negative value if you are behind UTC.
Altitude: The altitude of the sun for which the comet's location is computed (default is -18°, see above).
Morning/evening visibility: Select if you want a diagram for the morning or evening visibility of the comet.
Hit the "Create diagram" button if you have changed the location, UTC offset, the sun's altitude and/or the date!
If you want to select another comet, hit the respective button — your selected location and other settings will the be remembered and used for the next comet as well!
Linking to images
You are welcome to use these diagrams for non commercial purposes. When you do, please, always provide a link to my comet list website: http://www.theusner.eu/astro/comets/comets.php
You can directly link to the current image using the code shown below the diagram.
The orbital elements are converted to celestial coordinates following the tutorial provided by Paul Schlyter.
Horizontal coordinates are determined using code from NASA's IDL astronomy library. Thanks go to Wayne Landsman and Chris O'Dell for writing that code (co_aberration.pro, co_nutate.pro, nutate.pro, co_refract.pro, eq2hor.pro).
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