Earth & Sky Basics
The point in the sky directly above an observer's head
What is the zenith?
The line that extends Earth's equator onto the celestial sphere.
What is the celestial equator?
The Greek word for "wanderer" that was given to objects whose paths were less regular in the sky.
What is "planet"?
The Earth-centered view of the universe, which was widely accepted until the European Renaissance.
What is the geocentric model?
The name for a noticeable star pattern within a constellation, such as the Big Dipper within Ursa Major.
What is an asterism?
The line where the sky and the ground appear to meet
What is the horizon?
The two points in the sky where an extension of Earth's axis of rotation would intersect the celestial sphere
What are the celestial poles?
The ancient Greek philosopher and mathematician who believed circles and spheres were "perfect forms" and suggested Earth should be a sphere.
Who was Pythagoras?
The apparent backward movement of a planet across the sky, caused by the motion of Earth.
What is retrograde motion?
The ancient civilization that used a calendar based on a 365-day year and kept track of the star Sirius.
Who were the Egyptians?
An imaginary sphere of immense radius with Earth at its center, on which all celestial objects appear to be located
What is the celestial sphere?
This is the name for the Sun's apparent path around the celestial sphere over the course of a year.
What is the ecliptic?
A measure of a celestial object's brightness as seen from Earth.
What is apparent magnitude?
The slow, conical wobble of Earth's axis that causes the celestial poles to shift over thousands of years.
What is precession?
This civilization kept careful records of "guest stars" (stars that suddenly flare up) and dark spots on the Sun.
Who were the Chinese?
The name of the star that appears to move the least in the northern sky because it's so close to the north celestial pole.
What is Polaris, the pole star?
The reason the ecliptic is tilted by 23.5° relative to the celestial equator.
What is the tilt of Earth's axis of rotation?
This Greek thinker was one of the first to propose that Earth moved around the Sun, but his idea was rejected by most ancient scholars.
Who was Aristarchus of Samos?
The apparent shift in the direction of a nearby star caused by the motion of an observer on the Earth.
What is stellar parallax?
The famous stone circle in Britain, built by ancient people to keep track of the motions of the Sun and Moon, dating back to 2800 BCE.
What is Stonehenge?
The name for the part of the sky where stars never set below the horizon, always appearing above it.
What is the circumpolar zone?
The 18-degree-wide belt in the sky, centered on the ecliptic, where the Sun, Moon, and planets are always found
What is the zodiac?
The Greek living in Alexandria who made the first fairly accurate determination of Earth's diameter using observations of the Sun.
Who was Eratosthenes?
The reason ancient astronomers were unable to observe stellar parallax
What is the immense distance to stars, making the parallax shift immeasurably small with the tools they had?
Cosmology is the study of this.
What is the basic structure and origin of the cosmos or universe?