Astronomy and related astronomical terms

Ptolemy (about A.D. 100 to about A.D. 170) was an Egyptian astronomer, mathematician, and geographer of Greek descent who flourished in Alexandria during the second century A.D.

In several fields his writings represent the culminating achievement of Greco-Roman science, particularly his geocentric (earth-centered) model of the universe now known as the Ptolemaic system.

Nothing is known about Ptolemy's life except what can be inferred from his writings. His first major astronomical work, the Almagest (from a hybrid of Arabic and Greek, "the greatest"), was completed about A.D 150 and contains reports of astronomical observations which Ptolemy made over the preceding quarter of a century.

The size and content of his subsequent literary production suggests that he lived until about A.D. 170.

1. A neutron star that rotates rapidly and emits a beam of radiation. 2. Celestial sources that emit pulses of energy at regular intervals, ranging from a few seconds to a few thousandths of a second. 3. A neutron star which produces regular pulses of electromagnetic radiation.

The smallest unit of electromagnetic energy at a given frequency.

A photon is a quantum of light.

1. A mysterious, "quasistellar", or star-like, object in a galaxy's core which is very small, very bright, and very distant.

Most quasars are strong sources of radio energy.

2. An exceptionally powerful, yet very compact extragalactic object, whose exact nature is still uncertain.

Observations suggest that quasars may be the hyperactive nuclei of galaxies, while theory indicates that the high luminosity which is 100 to 1,000 times that of normal galaxies, could be associated with a supermassive black hole.

Radio signals transmitted to and bounced back from an object.

RADAR is an acronym for RAdio Detecting And Ranging.

The bouncing of radio waves off objects in the solar system, with reception and analysis of the "echoes".

Radar contact with the moon was first made in 1945 and with Venus in 1961. The travel time to radio reflections allows the distances of objects to be determined accurately.

Analysis of the reflected beam reveals the rotation period and allows the object's surface to be mapped. The rotation periods of Venus and Mercury were first determined by radar. Radar maps of Venus were obtained first by earth-based radar and subsequently by orbiting space probes.

The apparent point on the celestial sphere from which meteors making up a shower seem to emerge, as a result of a perspective effect.

Energy transmitted through space as waves or particles.

A subfield of astronomy that studies celestial objects at radio frequencies; a frequency that is useful for radio transmission, usually between 10 kHz and 300,000 MHz.

In addition to celestial bodies that radiate visible light, the universe contains many objects that emit radiation of various non-visible wavelengths; such as, radio waves.

Some astronomical objects emit very little light or even none at all; however, they may radiate relatively large amounts of energy at non-visible wavelengths.

Radio observations of celestial objects can be made from the earth's surface during the day and at night, both in cloudy and clear weather.

An instrument used to make observations of celestial bodies at radio wavelengths.

The equipment usually consists of an aerial which collects the radiation and feeds it to a processing computer.

A form of electromagnetic radiation traveling at the speed of light whose frequency lies between about 10 kilohertz and about 100,000 megahertz.

Radio waves are low frequency and, therefore, long wavelength electromagnetic radiations.

The radio part of the electromagnetic spectrum lies beyond the infrared region and comprises all radiation with a frequency lower than about 300,000 million hertz (300,000 MHz) or with a wavelength longer than about one millimeter.

Movement of a celestial object away from the observer, or from another celestial object.

A small, relatively cool star with low luminosity.

A star with a relatively low surface temperature, a few thousand degrees at most and radius between 10 and 100 times that of the sun.

Such objects are representative of stars at the end of their evolutionary life.

1. The observed shift of the characteristic spectral lines of, for example, a galaxy, toward the red, longer-wavelength, end of the spectrum as a result of the galaxy's recession from the earth.
2. The lengthening of the wavelengths of light from an object as a result of the object's motion away from the earth.

It is an example of the Doppler effect. The red shift in light from galaxies is evidence that the universe is expanding.