Astronomy Chapter S1

The celestial sphere is an imaginary sphere of infinite radius centered on Earth, with the North and South Celestial Poles defined by the extensions of the Earth's North and South Poles.
The celestial equator is the projection of the Earth's equator onto the celestial sphere, dividing it into the Northern and Southern Hemispheres.
The celestial sphere can be divided into a grid of latitude and longitude, similar to the Earth's coordinate system.
Right Ascension (RA) and Declination (Dec) are the celestial equivalents of longitude and latitude, used to locate objects in the sky.
RA is measured in hours, minutes, and seconds, while Dec is measured in degrees, arcminutes, and arcseconds.

Angular size is the angle subtended by an object at the observer's eye, measured in degrees, arcminutes, and arcseconds.
Angular size depends on the physical size of the object and its distance from the observer.

The Moon and Sun have nearly the same angular size, about 0.5 degrees.
The Moon's distance from Earth is about 380,000 km, while the Sun's distance is about 150,000,000 km.

The horizon is the flat plane where the observer stands, while the zenith is the point directly above the observer.
The nadir is the point opposite the zenith, directly below the observer.
An observer can see only half of the celestial sphere from any location on Earth.

The Earth's rotation from west to east makes the celestial sphere appear to rotate from east to west.
Depending on the observer's location, some stars will rise and set, while others will remain circumpolar.

The zodiac constellations are the 12 constellations through which the Sun, Moon, and planets appear to pass as the Earth orbits the Sun.
There are actually 13 zodiac constellations, including Ophiuchus, which is often overlooked.
The Sun spends different amounts of time in each constellation, due to their varying sizes.

The Earth's axis tilt of 23.5 degrees is the main cause of the seasons.
The distance between the Earth and the Sun varies by about 3% over the course of the year, but this has a negligible effect on the seasons.
The orientation of the Earth's axis changes over time due to precession, which takes about 26,000 years to complete.

The Moon rises in the east and sets in the west, moving across the sky in an arc.
The Moon moves slowly eastward against the stars, completing one orbit every 27.3 days.
The Moon's elliptical orbit causes its angular diameter to vary, appearing larger when it is at perigee (closest to Earth) and smaller when it is at apogee (farthest from Earth).

The Moon's phases change as it orbits the Earth, due to the changing angle between the Moon, Earth, and Sun.
The Moon's phases can be new, full, crescent, gibbous, or a combination of these.
The time it takes the Moon to complete a full cycle of phases is about 29.5 days, called the synodic period.

Eclipses occur when the Earth or Moon passes through the other's shadow.
Lunar eclipses occur at full moon, while solar eclipses occur at new moon.
There are three types of lunar eclipses: total, partial, and penumbral, and three types of solar eclipses: total, partial, and annular.### Solar Eclipses
The Moon's orbit is tilted 5° to the ecliptic plane, which is why we don't have an eclipse at every new and full moon.
Two eclipse seasons occur each year, with a lunar eclipse at new moon and a solar eclipse at full moon.

Two conditions must be met for an eclipse to occur:
- It must be full moon (for a lunar eclipse) or new moon (for a solar eclipse).
- The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes).

Eclipses recur with the 18-year, 11 1/3-day saros cycle, but the type (e.g., partial, total) and location may vary.
The next total solar eclipse seen from the continental US will be on August 21, 2017.

We see phases of the Moon because half of it is lit by the Sun, while half is in shadow, and its appearance to us is determined by the relative positions of the Sun, Moon, and Earth.
A lunar eclipse occurs when the Earth's shadow falls on the Moon, while a solar eclipse occurs when the Moon's shadow falls on the Earth.
The tilt of the Moon's orbit means that eclipses do not occur for every new or full moon, but rather during two periods each year.