A new era in the exploration of the planets began when Galileo focused his primitive telescope on the Moon. No longer were the planets mere points of light, but other worlds. The emphasis in planetary astronomy shifted from explaining the planets' motions to studying their natures.
As better telescopes were developed, astronomers produced more accurate maps of the Moon than the sketches of Galileo's day. This is a copy of the earliest known photograph of the Moon, a daguerreotype taken in 1851. The development of photography was important in astronomy. Still, the human eye can discern more detail than could be captured on a photographic plate.
Radar astronomy was born in 1946, when a radio signal was sent from Earth and "bounced" off the surface of the Moon. Radar was first reflected off the surface of Venus in 1961. This 1988 radar map of the surface of Venus was produced by the Arecibo radio telescope in Puerto Rico.
All objects with a temperature above absolute zero emit some radiation. Objects hotter than about 1,000° C (1,800° F) emit radiation in the visible wavelengths -- light humans see. Most planetary bodies, however, are cooler than a few hundred degrees and emit very short radio waves (microwaves). The amount of microwave energy emitted by a planet is a measure of its temperature. These microwaves can be detected by sensitive radio telescopes with large antennas.