Civil Dawn/Dusk = The Sun is between 6 degrees below the horizon and sunrise/sunset. This twilight phase is the brightest. As the Earth's atmosphere scatters and reflects much of the Sun's rays, coloring the sky bright yellow and orange, artificial lighting is generally not required in clear weather conditions to carry out most outdoor activities. In Houston civil twilight starts about 25-30 minutes before/after sunrise/sunset, depending on the time of year.

Nautical Dawn/Dusk = The moment when the geometric center of the Sun is between 12 and 6 degrees below the horizon. Many of the brighter stars can also be seen, making it possible to use the position of the stars in relation to the horizon to navigate at sea. In Houston nautical twilight starts an hour before/after sunrise/sunset, depending on the time of year.

Astronomical Dawn/Dusk = The geometric center of the Sun's disk is between 12 and 18 degrees below the horizon. To the naked eye, and especially in areas with light pollution, it may be difficult to distinguish astronomical twilight from night time. Most stars and other celestial objects can be seen during this phase. However, astronomers may be unable to observe some of the fainter stars and galaxies as long as the Sun is less than 18 degrees below the horizon—hence the name of this twilight phase. In Houston astronomical twilight starts about an hour and 25 minutes before/after sunrise/sunset, depending on the time of year.

FOV = Field of View.

Apparent Field of View = A constant for any paired eyepiece and telescope measured in degrees. The larger the AFov the more area of sky one can see. Eyepieces come with specific apparent fields of view, generally between 40 and 50 degrees. Eyepieces with an AFoV of 60 degrees or more are very expensive.

Actual Field of View = This is how much actual sky one can see measured in degrees. This is a function of the magnification of the paired eyepiece and telescope. The higher the magnification the smaller the field of view.

Magnitude = This is a measurement of stellar brightness. The brighter the star/planet the smaller the number. Each step of 1.0 in magnitude is a factor of 2.512, so a star magnitude of 3 is 2.512 times brighter than a start magnitude of 4. Magnitudes down to a 6 can be viewed with the naked eye. With 7x50 binoculars one can see down to a magnitude of about 10.5. Objects brighter than 0 have a negative numerical value.

Apparent Magnitude = This is how bright a star appears regardless of how bright it really is.

Absolute Magnitude = This is how bright a star really is, measured as the apparent magnitude if it were 32.616 light years away. This can be used to compare stellar bodies. For example, the sun's apparent magnitude is -26 and an absolute magnitude of +4.7. Daneb has an absolute magnitude of -8.73, which is more than 250,000 times the magnitude of the sun. Since it's 3,200 lightyears away, it has an apparent magnitude of +1.25.