Highlights of the History of Astronomy and the Basics of Solar System Astronomy

Historical:

• The ancient Babylonians kept vast written records of astronomical data.
• Since the 7th century B.C., the Chinese kept highly accurate records of astronomical data.
• Plato and Aristotle formulated a geocentric model of the solar system describing the Earth as the center, with the sun and other planets revolving around the Earth in perfect circular orbits and the planets simultaneously having epicyclical orbits about their solar orbits to explain "retrograde motion" of the planets observed from Earth.
• In the 2nd century B.C., Hipparchus improved the geocentric model slightly, made the first (relatively) precise observations of stars, and developed the apparent magnitude scale measuring the apparent brightness of a star on a scale of 1 (brightest) to 6 (faintest).
• In the 3rd century B.C., Aristarchus was the first to propose a heliocentric model of the solar system (in which the sun is the center, and the planets revolve around the sun) but his work was largely ignored.
• Ptolemy's sophisticated and far more detailed improvement on the geocentric model was so successful that it remained the near-universally accepted model of the solar system until the Renaissance.
• During the Renaissance, Copernicus developed a detailed heliocentric model of the solar system utilizing a great deal of mathematics that successfully predicted the astronomical patterns of solar-body movements, and whose work was built upon by Galileo Galilei and Johannes Kepler.
• Galileo, who developed the first relatively powerful telescope of the time, was the "father" of observational astronomy, and through his telescopically-collected observations discovered four moons orbiting Jupiter, that the moon had craters, the sun had "sunspots" (which he happened to correctly explain the nature of), observed that Venus (from the perspective of Earth) went through luminary phases similar to the moon, and used his observational evidence to directly support the validity of the heliocentric model.
• Later, Kepler used the couple decades of accurate solar observational data collected by Tycho Brahe and his own intellectual conceptual insights to develop his three law's of planetary motion (1. The orbit of a planet is an ellipse with the Sun at one of the two foci, 2. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time, 3. The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit).
• Then came along Isaac Newton, who developed the "universal law of gravitation," which very precisely explained mathematically the heliocentric model movements of solar-bodies and made extremely accurate predictions for the future motions and positions of solar-bodies.

Non-Historical:

• Only approximately a maximum of 2,000 stars are visible with the naked eye.
• The Earth's average diameter is 7,918 miles.
• The solar system has a diameter of approximately 9 trillion miles.
• 1 lightyear ≈ 5.88 trillion miles.
• The radius of the observable universe is approximately 14 billion lightyears.
• The apparent encompassing sphere of the heavens about the Earth is called the "celestial sphere."
• 88 constellations are officially recognized by the IAU; that star patterns not formally considered constellations are called "asterisms."
• The apparent path of the sun across the celestial sphere is called the "ecliptic."
• Modern astronomers use a "photometer" and an apparent brightness scale that ranges from -30 to +31? (+31 was assigned to the faintest object detected by the Hubble telescope) {each increment = a ratio of 2.5 from the next increment in degree of brightness}, the Sun measures -26.75 on this scale, and the next brightest star, Sirius, measures in at -1.44.
• Astronomers use "declination" (the angular measure above or below the celestial equator) and "right ascension" (angular measure in units of "hour" {1 hour = 1.5°} relative to the "celestial meridian" (0° in longitude) to define the location of astronomical objects.
• The point directly above (90° perpendicular to the horizon at your current location) is called the 'zenith.'
• A star is said to "culminate" (reaches its highest altitude in the sky) when it is on the celestial meridian.
• The daily paths of stars around the celestial axis are called "diurnal circles."
• To find the "North Star," Polaris, you can use the 'pointer' stars, Dubhe and Merak (which are at the end of the 'bowl' of the Big Dipper).
• The stars that never 'set' below the celestial horizon in the Northern/Southern hemisphere are called "North (or) South circumpolar stars" respectively.
• The band about 16° wide around the sky centered on the 'ecliptic' is called the 'zodiac,' which ancient astrologers divided into 12 constellations, or "signs," each taken at 30° increments of longitude.
• The Earth’s geographical equator remains tilted at about 23.4-23.5° to its orbital plane about the sun.
• The 'vernal equinox,' which occurs about March 20, is the Sun’s position as it crosses the celestial equator going north and is the point on the celestial sphere chosen to be the 0h measure of right ascension.
• The 'autumnal equinox,' which occurs about September 23, is the Sun’s position as it crosses the celestial equator going south.
• During the equinoxes, day and night are equal in length.
• The 'summer solstice,' which occurs about June 21, and the 'winter solstice,' which occurs about December 21, are the most northern and most southern positions of the Sun during the year in the northern hemisphere, and that at these times we have the longest and shortest days, respectively, in the northern hemisphere.
• There are two measures of "day": the 'solar day' (measuring the time interval of Earth’s rotation using the Sun for reference) and the 'sidereal day' (measuring the time interval of Earth’s rotation using the stars for reference).
• A sidereal day is 23 hours, 56 minutes, 4 seconds long, requiring the use of leap-years and leap-seconds.
• The Earth’s axis of rotation shifts extremely slowly around a imaginary 'cone' in space once about every 25,800 years (which is called "precession") and is caused mainly by the tug of the gravity of the Sun and Moon on Earth’s equatorial bulge.
• The Arctic Circle and Antarctic Circle mark the southernmost and northernmost latitude, respectively, at which the sun can remain continuously above or below the horizon for 24 hours (at the June solstice and December solstice respectively).
• The Tropic of Cancer and Tropic of Capricorn are the northern and southern latitudes, respectively, at which the sun reaches zenith only one time a year.