ESC 1000 Earth Science 

Chapter 21 Vocabulary

Fill in the blanks with the word(s) that best fit the description:


1. Aphelion – the place in the orbit of a planet where the planet is farthest from the Sun


Astronomical unit (AU) – Average distance from the earth to the Sun; 1.5 x 108 km or

 93 x 106 miles


2. Celestial sphere – An imaginary hollow sphere upon which the ancients believed the stars were hung and carried around the earth


3. Constellations – An apparent group of stars originally names fir mythical characters. The sky is presently divided into 88 of the these groups.


4. Declination –The angular distance north or south of the celestial equator denoting the position of a celestial body.


5. Ecliptic – The yearly path of the Sun plotted against the background of the stars.


Equatorial system – A method of locating stellar objects much like the coordinate system used on the Earth’s surface.


Geocentric – The concept of an Earth-centered universe.


6. Heliocentric – The view that the Sun is at the center of the solar system.


7. Lunar eclipse – An eclipse of the moon.


8. Mean solar day – The average time between two passages of the Sun across the local celestial meridian.


9. Perihelion – The point in the orbit of a planet where it is closest to the sun.


10. Perturbation – The gravitational disturbance of the orbit of one celestial body by another


Phases of the moon – The progression of chances in the Moon’s appearance during the month.


11. Plane of the ecliptic - The imaginary plane that connects Earth’s orbit with another celestial sphere


12. Precession -  A slow motion of Earth’s axis that traces out a cone over a period of 36,000 years


Ptolemaic system – An Earth-centered system of the universe.


Retrograde motion – The apparent westward motion of the planets with respect to the stars.


13. Revolution - The motion of one body about another, as the Earth about the sun.


14. Right ascension – An angular distance measured eastward along the celestial equation from the vernal equinox


15. RotationThe spinning of a body such as the Earth, about its axis.


Sideral day – The period of Earth’s rotation with respect to the stars.


Sideral month – A time period based on the revolution of the Moon around the earth with respect to the stars.


16. Solar eclipse – An eclipse of the Sun


 Synodic month – The period of revoltion of the Moon with respect to the Sun, or its cycle of phases.