An antenna for receiving radio waves, consisting of a small number of parallel dipoles. The commonest domestic television aerials are of this type. Arrays of Yagis are used in radio astronomy to construct cheap aperture synthesis telescopes for sky survey work. See also: radio telescope.

The effect of its rotation on the path of a small particle orbiting the Sun. Rotation causes a temperature variation, so thermal energy is re-radiated anisotropically. See also: Poynting-Robertson effect.

The period of time taken for the Earth to orbit the Sun. The exact length of the year depends on the reference point taken. See also: calendar, calendar year.

Year	Reference point	Length in days
Tropical	Equinoxes	365.242 19
Siderial	Fixed stars	365.256 36
Anomalistic	Apsides	365.259 64
Eclipce	Moon’s node	346.620 03
Gaussian	Kepler’s low for semimajor axis of 1 AU	365.256 90

An observatory in Williams Bay, Wisconsin, which belongs to the Department of Astronomy and Astrophysics of the University of Chicago. The telescope housed at the observatory is the largest refracting telescope ever built, with an objective lens 1 metre 40 inches in diameter. It was constructed between 1895 and 1897. The telescope was largely the brainchild of George Ellery Hale, who later became the driving force behind the 2.5-metre 100-inch Hooker Telescope at Mount Wilson and the 5-metre 200-inch Palomar telescope. In association with William Rainey Harper, he persuaded the Chicago millionaire Charles Yerkes to finance the entire project. The lens components were already in existence, having been manufactured for another project that never came to fruition. The mechanical parts of the telescope were constructed by Warner and Swasey of Cleveland, Ohio. The building is noted for its ornate design by the architect Henry Ives Cobb. The 40-inch telescope is still used in some of the Department’s research programmes.

The primeval fireball.

The name given after its launch in August 1991 to a Japanese astronomy satellite originally designated Solar-A. Its prime purpose was the study of X-rays and gamma rays from solar flares and other energetic phenomena on the Sun.

A particular form of equatorial mounting.

A member of a subclass of T Tauri stars characterized by a particular form of emission line with an absorption wing to the red side. The line profiles are variable over periods of days. About half of all T Tauri stars may belong to this subclass.