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The Sun is the star at the center of the Solar System.It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light and infrared radiation with 10% at ultraviolet energies.
Since the axial tilt of Earth is considerable (23 degrees, 26 minutes, 21.41196 seconds), at high latitudes the Sun does not set in summer; [8] rather, it remains continuously visible for one day during the summer solstice at the polar circle, for several weeks only 100 km (62 mi) closer to the pole, and for six months at the pole. At extreme ...
Since the Earth rotates at a mean speed of one degree every four minutes, relative to the Sun, this 16-minute displacement corresponds to a shift eastward or westward of about four degrees in the apparent position of the Sun, compared with its mean position. A westward shift causes the sundial to be ahead of the clock.
Angles in the hours ( h), minutes ( m), and seconds ( s) of time measure must be converted to decimal degrees or radians before calculations are performed. 1 h = 15°; 1 m = 15′; 1 s = 15″ Angles greater than 360° (2 π ) or less than 0° may need to be reduced to the range 0°−360° (0–2 π ) depending upon the particular calculating ...
At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360 degrees and the distant star is overhead again but the Sun is not (1→2 = one stellar day). It is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day).
The third phase is astronomical twilight, which is the period when the Sun is between 12 and 18 degrees below the horizon. [2] Dusk is at the very end of astronomical twilight, and is the darkest moment of twilight just before night. [3] Finally, night occurs when the Sun reaches 18 degrees below the horizon and no longer illuminates the sky. [4]
In the year −2000 (2001 BCE) the May maximum was +12 minutes and a couple seconds while the November maximum was just less than 10 minutes. The secular change is evident when one compares a current graph of the equation of time (see below) with one from 2000 years ago, e.g., one constructed from the data of Ptolemy.
For example, a sundial designed for a latitude of 40° can be used at a latitude of 45°, if the sundial plane is tilted upwards by 5°, thus aligning the style with the Earth's rotational axis. [citation needed] Many ornamental sundials are designed to be used at 45 degrees north.