Sponge: Why is the “Principle of Mediocrity” necessary to the study of astronomy?

Ancient astronomers believed that the stars were attached to a celestial sphere surrounding the Earth.

Celestial sphere - a canopy of stars that appears as though the stars were painted on an imaginary ceiling.

Locations of stars and planets in the sky is simplified with a system of celestial coordinates.

Celestial coordinates projects the familiar system of latitude and longitude on the inside of the celestial sphere.

The north celestial pole is directly above the Earth’s north pole The north celestial pole is directly above the Earth’s north pole. The south celestial pole is directly above the Earth’s south pole.

The celestial equator is halfway between the poles and is directly above Earth’s equator.

Declination is like latitude Declination is like latitude. It is measured in degrees north or south of the celestial equator.

The declination (abbreviated dec) of the celestial equator is 0°.

The north celestial pole is +90°, and the south celestial pole is -90°.

Right ascension (RA) is like longitude Right ascension (RA) is like longitude. It is measured in hours, minutes and seconds.

One hour (1h) is 15°, one minute (1m) is 1/60 of an hour, and one second (1s) is 1/60 of a minute.

Zero right ascension is arbitrarily designated as a semicircle that passes a point in the sky midway between the constellations Pisces and Aquarius.

Daily motion of the Sun and stars across the sky is called diurnal motion.

A day measured by the position of the Sun is called a solar day.

A day measured by the position of the stars is called a sidereal day.

The solar day is 3.9 minutes longer than a sidereal day.

The Sun’s path through the celestial sphere forms an ecliptic to the celestial sphere at an angle of 23.5 °.

This angle (23.5°) is a result of the Earth’s tilt on it’s axis.

A tropical year (the time for the Sun to complete one trip around the ecliptic) is 365.242 solar days.

The northernmost point above the celestial equator (dec +23 The northernmost point above the celestial equator (dec +23.5°) is called the summer solstice. (occurs around June 21)

The southernmost point below the celestial equator (dec -23 The southernmost point below the celestial equator (dec -23.5°) is called the winter solstice. (occurs around December 21)

The summer solstice is also the longest day of the year in the northern hemisphere (the winter solstice is the shortest).

This combination (height of the Sun, length of day) accounts for our seasons.

The two points where the ecliptic crosses the celestial equator are called the equinoxes.

In the fall - autumnal equinox (Sept 22).

In the spring - vernal equinox (March 21)

The vernal equinox is said to have a RA of 0°, which is between Pisces and Aquarius.