![]() Astronomical Twilight Astronomical twilight is the darkest of the three twilight phases. Winter: December solstice to March equinox.Fall (autumn): September equinox to December solstice.Summer: June solstice to September equinox.Spring: March equinox to June solstice. ![]() In the Northern Hemisphere, the four astronomical seasons are: Astronomical Season Astronomers and scientists use the dates of equinoxes and solstices to mark the beginning and end of seasons in a year. Their size ranges from a few meters to hundreds of kilometers in diameter. Asteroid Asteroids are rocky objects orbiting the Sun. Its stars belong to the constellation Ursa Major. The Big Dipper is one of the most well-known asterisms. Asterisms may or may not be part of a constellation. Asterism A pattern of stars recognizable to observers from Earth. Apparent Retrograde Motion Apparent retrograde motion is where a superior planet appears to change direction in the sky for a short time as it is overtaken by Earth in its orbit around the Sun. Aphelion Aphelion is the point farthest away from the Sun on Earth's elliptical orbit. It is involved in annular solar eclipses and planet transits. Antumbra The antumbra is the lighter part of a shadow that forms at a certain distance from the object casting the shadow. Annular Solar Eclipse An annular solar eclipse happens when the Moon covers the Sun's center, leaving the Sun's visible outer edges to form a “ring of fire” or annulus around the Moon. Altitude (elevation) Altitude or elevation is the vertical angle an object makes with the horizon. The Moon has an Albedo of 0.12, while Earth's average albedo is 0.3. 1 on the scale refers to a perfectly reflective object. Albedo is measured on a scale from 0 to 1, where 0 refers to an object that is completely dark, i.e., it does not reflect any light. Derived from Latin, albedo means "whiteness" ( albus= "white"). Top Albedo In astronomy, the term albedo refers to the brightness of an object in space. Business Date to Date (exclude holidays).
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