The Old Farmers Almanac - 2009 - Outdoors (Page 96)

Astronomical Glossary

Aphelion (Aph.): The point in a planet’s orbit that is farthest from the Sun.

Apogee (Apo.): The point in the Moon’s orbit that is farthest from Earth.

Celestial Equator (Eq.): The imaginary cir-
cle around the celestial sphere that can be
thought of as the plane of Earth’s equator pro-
jected out onto the sphere.
^Celestial ^Sphere:^An ^imaginary ^sphere ^pro-^Ecliptic:^The ^apparent ^annual ^path ^of ^{the jected} ^into ^space ^that ^represents ^the ^{entire Sun} ^around ^the ^celestial ^sphere. ^The ^plane ^ofthe ecliptic is tipped 231⁄ 2° from the celestial
^sky, ^with ^an ^observer ^on ^Earth ^at ^its ^center.All celestial bodies other than Earth are imag- ^equator.ined as being on its inside surface. Elongation: The difference in degrees be-
tween the celestial longitudes of a planet and

^Conjunction:^The ^time ^at ^which ^two ^or ^morethe Sun. Greatest Elongation (Gr. Elong.): celestial bodies appear closest in the sky. Infe- _The _greatest _apparent _distance _of _a _planet^rior ^(Inf.):^Mercury ^or ^Venus ^is ^between ^thefrom the Sun, as seen from Earth.

Sun and Earth. Superior (Sup.): The Sun is between a planet and Earth. Actual dates for Epact: A number from 1 to 30 that indicates conjunctions are given in the Right-Hand Calen- the Moon’s age on January 1 at Greenwich, dar Pages, 113–139; the best times for viewing England; used for determining the date of the closely aligned bodies are given in Sky Watch Easter. ^on ^the ^Left-Hand ^Calendar ^Pages, ^112–138^. Equinox: When the Sun crosses the celestial Declination: The celestial latitude of an ob- equator. This event occurs two times each ject in the sky, measured in degrees north or ^year: ^Vernal^is ^around ^March ²⁰ ^and ^Autum-south of the celestial equator; analogous to nal ^is ^September ²² ^or ²³^.latitude on Earth. This Almanac gives the _Evening _Star:A planet that is above the ^Sun’s ^declination ^at ^noon.western horizon at sunset and less than 180° _Eclipse, _Lunar:The full Moon enters the east of the Sun in right ascension. shadow of Earth, which cuts off all or part of Golden Number: Anumber in the 19-year cy- ^the ^sunlight ^reflected ^off ^the ^Moon. ^Total:^Thecle of the Moon, used for determining the date ^Moon ^passes ^completely ^through ^the ^umbraof Easter. (Approximately every 19 years, the (central dark part) of Earth’s shadow. Partial: Moon’s phases occur on the same dates.) Add ^Only ^part ^of ^the ^Moon ^passes ^through ^the ^um- 1 to any given year and divide by 19; the re- ^bra. ^Penumbral: ^The ^Moon ^passes ^throughmainder is the Golden Number. If there is no ^only ^the ^penumbra^(area ^of ^partial ^darknessremainder, the Golden Number is 19. surrounding the umbra). See page 88 for more _eclipse _information.Greatest Illuminated Extent (Gr. Illum.

Ext.): When the maximum surface area of a

^Eclipse, ^Solar:^Earth ^enters ^the ^shadow ^ofplanet is illuminated as seen from Earth. the new Moon, which cuts off all or part of _the _Sun’s _light. _Total:_Earth _passes _throughJulian Period: A period of 7,980 years begin- _the _umbra _(central _dark _part) _of _the _Moon’sning January 1, 4713 B. C. It provides a chrono- _shadow, _resulting _in _totality _for _observerslogical basis for the study of ancient history. _within _a _narrow _band _on _Earth. _Annular: _TheTo find the Julian year, add 4,713 to any year. Moon appears silhouetted against the Sun, Magnitude: A measure of a celestial ob-with a ring of sunlight showing around it. ject’s brightness. Apparent magnitude Partial: The Moon blocks only part of the Sun. measures the brightness of an object as seen