| Back Focal Length |
The distance from the final surface of an optical system to the focal plane. |
| Bandpass |
The selectivity of a filter. |
| Barley Moon |
The name of the Full Moon usually in September, if the September Full Moon is not the Harvest Moon. Specifically the first Full Moon before the Autumnal Equinox (September 23 by "The Maine Rule"). Sometime known as the "Fruit Moon" or "Barley Moon." |
| Barlow Lens |
A small negative lens that amplifies and relocates a telescope's beam when placed just inside focus. It is used to obviate the use of short-focus eyepieces and in negative lens projection photography. |
| Beaver Moon |
The name of the Full Moon usually in November, if the November Full Moon is not the Hunters Moon. Specifically the second Full Moon before the Autumnal Equinox (September 23 by "The Maine Rule"). Sometimes known as the "Frosty Moon." |
| Belinda |
A moon of Uranus was discovered by Stephen P. Synnott (Voyager 2) on January 13, 1986. Belinda is also designated as "Uranus XIV" and "S/1986 U5". Belinda is the heroine of Alexander Pope's The Rape of the Lock. |
| Bianca |
A moon of Uranus was discovered by Bradford A. Smith (Voyager 2) on January 23, 1986. Bianca is also designated as "Uranus VIII" and S/1986 U9". Bianca is the sister of Katherine (Kate) in Shakespeare's play The Taming of the Shrew. |
| Binary Star |
Two stars revolving about each other; a double star. |
| Blue Moon |
A "Blue Moon" was originally defined as the 3rd Full Moon in an season where 4 Full Moons occur. This is the "Maine Rule" as it was used by the Maine Farmers' Almanac. Due to a misinterpretation in a 1946 article in Sky & Telescope of on a 1943 Sky & Telescope article, a definition emerged of a "Blue Moon" being the second Full Moon in a calendar month. This simpler and popular rule is known as the "Sky & Telescope Rule." The Maine Rule uses the circular orbit of a fictitious mean Earth to define the seasons. |
| Bootes |
The constellation "The Herdsman" |
| Boundary Layer, Thermal |
The layer of relatively warm air on a telescope objective that is cooling down. The layer becomes harmful to definition if it becomes unstable, as by agitation in open-tube telescopes. |
| Buck Moon |
The name of the Full Moon usually in July. Specifically the first Full Moon after the Summer Solstice (June 22 by "The Maine Rule"). Sometimes known as the "Thunder Moon" or the "Hay Moon." |
