The Essential Cosmic Perspective: Chapter 2

Constellation

a region in the sky

celestial sphere

an imaginary sphere that the stars and constellations lie on

north celestial pole

the point directly over Earth's North Pole

South Celestial pole

the point directly over Earth's South Pole

celestial equator

projection of Earth's equator into space, makes a complete circle around the celestial sphere

ecliptic

the path the sun follows as it appears to circle around the celestial sphere once each year. it crosses the celestial equator at 23.5 angle, because that is the tilt of Earth's axis.

local sky

the sky as seen from wherever you happen to be standing

horizon

the boundary between the Earth and the Sky

zenith

the point that lies directly overhead

meridian

imaginary half-circle stretching from the horizon due south, through the zenith, to the horizon due north.

angular size

A measure of the angle formed by extending imaginary lines outward from our eyes to span an object (or the space between two objects).

angular distance

the angle that appears to separate 2 objects

circumpolar

stars near the north celestial pole that do not rise or set. they make counterclockwise circles around the north star.

latitude

measures north-south positions

longitude

measures east-west positions

summer (June) solstice

June 21, is the moment when the Northern Hemisphere is tipped most directly toward the Sun

winter (December) solstice

December 21, is the moment when the Northern Hemisphere is tipped most directly away from the Sun

spring (March) equinox

March 21, is the moment when the Northern Hemisphere goes from tipped slightly away from the Sun to being tipped slightly toward the Sun.

fall (September) equinox

occurs around September 22, the moment when the Northern Hemisphere first stars to be tipped away from the sun.

precession

a gradual wobble that changes the orientation of Earth's axis in space

eclipse

when the earth sun and moon fall into a straight line

lunar eclipse

the earth lies directly between the sun and the moon, so that the earth's shadow falls on the moon

solar eclipse

moon lies directly between the sun and earth so that the moon's shadow falls on Earth. people in the shadow will see the sun blocked.

umbra

central area where light is blocked from the shadow of the moon or earth from an eclipse

penumbra

area where light is partially blocked from the shadow of the moon or earth from an eclipse

total lunar eclipse

if the sun, earth, and moon are nearly perfectly aligned

partial lunar eclipse

less perfect alignment of the sun earth and moon. only part of the full moon will pass through the umbra (with the rest in the penumbra) we will see a...

penumbral lunar eclipse

if the moon passes only through earth's penumbra

totality

the time during which the moon is entirely engulfed in the umbra

total solar eclipse

the moon is relatively close to Earth in it's orbit, the moon's umbra touches a small area of earth's surface

partial solar eclipse

only part of the sun is blocked from view

annular eclipse

a ring of sunlight surrounding the moon

apparent retrograde motion

occasionally planets reverse course, moving west to east through the zodiac (usually planets move east to west)

stellar parallax

The apparent shift in the position of a nearby star against a backdrop of distant, fixed stars caused by Earth's orbital motion around the Sun