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Astronomy & Telescopes Glossary - D
Dark adaptation - Changes in your eyes that allow you to see well in the dark. It takes about 20 minutes for your eyes to completely adapt to darkness.
Declination - Similar to latitude on the Earth's surface. The distance in degrees north or south of the celestial equator (similar to the Earth's Equator). Declination is measured in degrees, minutes and seconds.
Degree - A measure of angle. There are 360 degrees in a circle. Each degree is divided into 60 minutes.
Deep sky - Any celestial object beyond the Solar System.
Dew cap - A tube attached to the forward end of a telescope used to prevent dew from forming on the lens as it cools down.
Diagonal - A telescope mirror or prism that changes the angle and orientation of light in an eyepiece.
Diamond ring effect - The burst of light that appears a few seconds before and after totality during a solar eclipse. It is caused by sunlight shining through valleys on the edge of the moon.
Diffuse nebula - A wide, irregularly-shaped cloud of gas that can be up to 100 light-years across.
Digital setting circles (DSC) - Electronic equivalent of a mechanical setting circles. These devices are used with an altazimuth telescope mounts and aid in aligning the telescope.
Dobsonian telescope - A Newtonian reflector telescope that sits in a special type of mount, called a Dobonsian mount. Dobsonian mounts sit on the ground and have a swiveling base. Dobsonian mounts are very stable, usually are made of wood, and reduce the cost of the telescope.
Double star - Two or more stars that appear very close in position. True double stars are in orbit about one another, while optical double stars simply appear close together as seen from the Earth.
DSC - Abbreviation for Digital Ssetting Circles.
Dust lane - A dust lane is lane of dark interstellar dust in a galaxy or emission nebula. Dust lanes obscure light.