study of celestial bodies by examination of the radio-frequency energy they emit or reflect. Radio waves penetrate much of the gas and dust in space, as well as the clouds of planetary atmospheres, and pass through Earth’s atmosphere with little distortion. Radio astronomers can therefore obtain a much clearer picture of stars and galaxies than is possible by means of optical observation. The...
lunar parallax determination
Radar and laser measures of the distance from Earth to the Moon have provided a recent value of the lunar parallax. Radar and laser ranges have the advantage of being a direct distance measure, although the ranges are affected by variations in the surface topography of the Moon and require assumptions about the lunar radius and the centre of mass. The International Astronomical Union in 1964...
Radar is also widely used for meteor detection and observation. Radar pulses (short bursts of radio-frequency energy) emitted from a ground-based transmitter are reflected by a meteor’s trail. The distance to the meteor can be determined by measuring the time lapse between the transmitted and reflected radar signals. Its motion toward (or away from) the radar facility can be derived from the...
...ways to determine these same variables. This technique has also been employed to remotely measure the properties of the plasmas in the near-space regions of the Earth using the incoherent scatter radar method. The largest single antenna is at the National Astronomy and Ionosphere Center at Arecibo in Puerto Rico. It has a circumference of 305 metres and was completed in 1963. It is still used...
solar parallax determination
Radar measures of the distance from Earth to Venus have provided the best determination of the solar parallax. By measuring the flight time of a radar pulse to Venus, the distance between the two planets can be obtained, allowing the determination of the unit distance between Earth and the Sun.
After finding that Venus is completely enshrouded by clouds, astronomers turned to other techniques to study its surface. Foremost among these has been radar. If equipped with an appropriate transmitter, a large radio telescope can be used as a radar system to bounce a radio signal off a planet and detect its return. Because radio...