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Glossary

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Term	Definition

Damocloid

a rare type of asteroid with an elliptical, comet-like orbit; named for the first one discovered, asteroid 5335 Damocles

dark adaptation

the process by which the human eye becomes well adjusted to seeing dim objects in the dark.

dark energy

a type of "negative gravity" that seems to play a role in the acceeleration of universal expansion

dark matter

matter that exerts gravitational force but does not emit any detectable light or radiation; dark matter comprises most of the mass of the universe but its exact nature remains unknown

dark nebula

a cloud of dust grains that is thick enough to obscure the light from background stars

declination

the angular distance of a celestial object above or below the celestial equator; the celestial sphere equivalent of latitude

deep-sky objects

objects located beyond the solar system; consist of stars, nebulae, star clusters, and galaxies.

degree

1. a unit of angular size equal to 1/360 the circumference of the celestial sphere; the sun and full moon both appear about half a degree wide

Degree Angular Scale Interferometer (DASI)

a 13-element microwave interferometer, located at the NSF Amundsen-Scott South Pole station, used to measure temperature and polarization in the Cosmic Microwave Background

Denison Olmsted

Denison Olmsted (1791-1859) is credited with giving birth to meteor science after the 1833 Leonid Meteor Storm over North America spurred him to study this phenomenon. He subsequently demonstrated that meteors are not an atmospheric phenomenon, but cosmic in origin.

Olmsted was born in East Hartford, Connecticut on June 18, 1791. He attended Yale University and graduated with a degree in physics in 1813. In 1817, he became the chair of chemistry, mineralogy, and geology at the University of North Carolina. He returned to Yale in 1825 where he was appointed professor of mathematics and natural philosophy.

Olmstead had many academic pursuits on which he published several textbooks and papers before his death in New Haven, Connecticut on May 13, 1859.

density

the amount of mass per unit volume of an object or region of space

deuterium

an isotope of hydrogen; its nucleus, consisting of one proton and one neutron, has double the mass of the nucleus of ordinary hydrogen

diffraction

the spreading out of light as it passes the edge of an obstacle

Dobsonian telescope

a telescope with a simple but stable altazimuth mount that rotates easily

Doppler effect

the change in wavelength of radiation coming from a source that’s moving toward or away from an observer; produces either a blueshift or redshift

dust	tiny particles floating in space

dwarf galaxy

a small galaxy containing a few million stars; the most common type of galaxy in the universe

dwarf star

a main-sequence or smaller star

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