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Usually, the term sky informally refers to a perspective from the Earth's surface; however, the meaning and usage can vary. An observer on the surface of the Earth can see a small part of the sky, which resembles a dome (sometimes called the sky bowl) appearing flatter during the day than at night. [1]
Sky conditions are estimated in terms of how many eighths of the sky are covered in cloud, ranging from 0 oktas (completely clear sky) through to 8 oktas (completely overcast). In addition, in the SYNOP code there is an extra cloud cover indicator '9' indicating that the sky is totally obscured (i.e. hidden from view), usually due to dense fog ...
[1] degree day Deviation of one-degree temperature for one day from an arbitrary standard, usually the long-term average temperature for a place. [2] dell A small, secluded hollow, usually within a grassy, park-like, partially wooded valley. delta A landform at the mouth of a river where the main stem splits up into several distributaries. It ...
On any given (sidereal) day/night, a celestial object will appear to drift across, or transit, the observer's upper meridian as Earth rotates, since the meridian is fixed to the local horizon. At culmination , the object contacts the upper meridian and reaches its highest point in the sky.
Zenith stars (also "star on top", "overhead star", "latitude star") [7] are stars whose declination equals the latitude of the observers location, and hence at some time in the day or night pass culminate (pass) through the zenith. When at the zenith the right ascension of the star equals the local sidereal time at your location.
where N is the day of the year beginning with N=0 at midnight Universal Time (UT) as January 1 begins (i.e. the days part of the ordinal date −1). The number 10, in (N+10), is the approximate number of days after the December solstice to January 1. This equation overestimates the declination near the September equinox by up to +1.5°.
The earliest known example is the one constructed by Crates of Mallus in Cilicia (now Çukurova in modern-day Turkey), in the mid-2nd century BC. No terrestrial globes from Antiquity have survived. An example of a surviving celestial globe is part of a Hellenistic sculpture, called the Farnese Atlas , surviving in a 2nd-century AD Roman copy in ...
Relationship of Earth's axial tilt (ε) to the tropical and polar circles. The Arctic Circle is the southernmost latitude in the Northern Hemisphere at which the centre of the Sun can remain continuously above or below the horizon for twenty-four hours; as a result, at least once each year at any location within the Arctic Circle the centre of the Sun is visible at local midnight, and at least ...