**Fig. 1.1 . **Values of the solar parallax obtained from measurements of the parallaxes of Venus, Mars, and the asteroid Eros between 1850 and 1970. The solar parallax, designated by p, is half the angular displacement of the Sun viewed from opposite sides of the Earth. The error bars denote the probable errors in the determination, and the points for 1941, 1950 and 1965 all have errors smaller than the plotted points. In the 1960s, the newly developed radar (radio detection and ranging) technology enabled the determination of the Suns distance with an accuracy of about 1000 meters. The radar value of the solar parallax is 8.79405 seconds of arc.

**Fig. 1.2 . **All forms of radiation consist of electric and magnetic fields that oscillate at right angles to each other and to the direction of travel. They move through empty space at the speed of light. The separation between adjacent wave crests is called the wavelength of the radiation and is usually designated by the lower case Greek letter lambda, l.

**Fig. 1.3 . **Radiation from cosmic objects can be emitted at wavelengths from less than 10-12 m to greater than 104 m, where m denotes meters. The visible spectrum that we see with our eyes is a very small portion of the entire range of wavelengths. Lighter shading indicates a greater transparency of the Earths atmosphere to cosmic radiation. It only penetrates to the Earths surface at visible and radio wavelengths, respectively represented by the narrow and broad white areas. Electromagnetic radiation at short gamma ray, x-ray and ultraviolet wavelengths, represented by the dark areas, is absorbed in our atmosphere, so the universe is now observed in these spectral regions from above the atmosphere in Earth-orbiting satellites.

**Fig. 1.4 . **The solar radius can be determined from the Suns angular size and distance. As long as this angle is small, the physical size is just a small arc of a large circle, denoted by the dashed line, and the angular size is the ratio of the physical size to the distance. Astronomers specify this angle as a partial arc of a full circle of 360 degrees, and for the Sun it is about 32 minutes of arc, where there are 60 minutes of arc in one degree. This angle has been enlarged to display it in this illustration. In mathematics, the radian is the standard unit of angular measure. It describes the angle subtended by a circular arc as the length of the arc divided by the radius of the arc. When the arc length is equal to the arc radius, the angle is one radian. You can convert between the two methods of describing angles by noting that the circumference of a circle is 2p times its radius, so 1 radian is equal to 360 degrees/(2p), or 57.2958 degrees. For the Sun, the angular size q = 2R/D radians, where R denotes the Suns radius and the mean distance of the Sun, D, is 1 AU. The observed angular size of the Sun corresponds to a radius of 695.5 million meters.

**Fig. 1.5 . **The spectral plot of black body radiation intensity as a function of wavelength depends on the temperature of the gas emitting the radiation. The German physicist Max Planck (1858-1947) derived the formula that describes the shape and peak of this spectrum in 1900, proposing that the radiation energy was quantized and providing a foundation for quantum theory. At higher temperatures the wavelength of peak emission shifts to shorter wavelengths, and the thermal radiation intensity becomes greater at all wavelengths. At a temperature of 6,000 degrees on the kelvin scale, or 6,000 K, the thermal radiation peaks in the visible, or V, band of wavelengths. A hot gas with a temperature of 100,000 K emits most of its thermal radiation at ultraviolet, or UV, wavelengths, while the emission peaks in x-rays when the temperature is 1 million to 10 million K.

**Fig. 1.6 . **Light waves that fall on the Earth from a distant object are parallel to one another, and they are focused to a point by the lens or mirror of a telescope. Early telescopes were refractors (left). The curved surfaces of the convex objective lens bend the incoming parallel light rays by refraction, and bring them to a focus at the center of the focal plane, where the light rays meet and an image is created. A second, smaller lens, called the eyepiece, was used to magnify the image in the early refractors; later versions placed photographic or electronic detectors at the focal plane. In 1670 the English physicist Isaac Newton (1643-1727) constructed the first reflecting telescope (right), which uses a large, concave, or parabolic, primary mirror to collect and focus light. A small, flat secondary mirror, inclined at an angle of 45 degrees to the telescope axis, reflects the light sideways, at a place now known as the Newtonian focus. Other light-deflecting mirror arrangements can be used to obtain any desired focal length, which varies with the curvature and position of small convex mirrors.

**Fig. 1.7 . **When incoming radiation approaches the Earth at an angle, the crests of the radiation will arrive at two separated telescopes at slightly different times. This delay in arrival time is the distance X divided by the speed of light. If X is an exact multiple of the wavelength, then the waves detected at the two telescopes will be in phase and add up when combined. If not, they will be out of phase and interfere. The angular resolution of such an interferometer, or interference meter, is equal to the wavelength divided by the effective baseline. When the object being observed is directly overhead, the effective baseline is equal to the distance between the two telescopes.