The darker the box, the higher the elevation and the easier to photograph
Unless you live on a wide-open expanse of flat land, you probably have trees, hills, buildings, or other things obscuring at
least part of the sky. Even if you have a clear 3600 view , the lower you look the more artifical
lighting and atmospheric absorption limits what you can see or capture through a telescope or camera.
If you live away from urban areas and have a clear view
of the horizon, then you generally want to be above 15°. If you are near an urban area with a lot of lights, then you want to be above 30°.
If you have trees, hills, or buildings, then you may only be able to see objects above 40-60°, depending on what the obstacles are, how high
they are, and how far away you are from them.
The visibility tables show what is visible for your location, and when, in a simple gray-scale format. The time from sunset to sunrise is divided
into 1-hour intervals, and the average position of all objects in the catalogs is calculated for each period; the darker the interval, the higher
the object. Objects that do not average higher than 300 are not shown, except for the moon. Even when the moon is low in the sky it can
adversely affect seeing conditions.
You can advance by days or months to help plan your observation campaign for a given week or season. For example, if M1, The Crab Nebula,
is not high enough until 4 in the morning, but next month will be high enough at 1, then you can decide when you want to to view it (stay up late
or get up early).
You can limit the magnitude of objects shown in the table, which can be useful depending on your location and equipment. Some objects in the catalogs
do not have assigned magnitudes, and you have the option to either hide or display them. If you are not sure which to choose, experiment and decide
what works best for you.
We use data from different sources, and therefore what data is shown may vary between tables.
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