The Old Farmer's Almanac 2015 (10 page)

Mercury

Mercury alternately darts above the eastern and western horizons roughly every 2 months. Observers can see this tiny world when it’s at least 5 degrees above the horizon and when it exceeds magnitude 0.5. This year, such favorable conditions occur in the western sky at dusk during the first half of January and from April 23 to May 7. In the predawn eastern sky, Mercury puts in a marginal appearance from February 20 to 26 and a much better one from October 8 to 27. Mercury forms a close conjunction with Venus from January 7 to 12.

DO NOT CONFUSE

• Venus and Jupiter in late June and early July.
  Venus is brighter.
  
• Jupiter, Mars, Regulus, and Venus vertically aligned in early October.
  They appear as given in order here, from lowest to highest.
  
• Mars and Virgo’s star Spica at year’s end.
  Spica is brighter and blue.
  

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

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

Celestial Equator (Eq.):
The imaginary circle around the celestial sphere that can be thought of as the plane of Earth’s equator projected out onto the sphere.

Celestial Sphere:
An imaginary sphere projected into space that represents the entire sky, with an observer on Earth at its center. All celestial bodies other than Earth are imagined as being on its inside surface.

Circumpolar:
Always visible above the horizon, such as a circumpolar star.

Conjunction:
The time at which two or more celestial bodies appear closest in the sky.
Inferior (Inf.)
: Mercury or Venus is between the Sun and Earth.
Superior (Sup.):
The Sun is between a planet and Earth. Actual dates for conjunctions are given on the
Right-Hand Calendar Pages
[>]
; the best times for viewing the closely aligned bodies are given in Sky Watch on the
Left-Hand Calendar Pages
[>]
.

Declination:
The celestial latitude of an object in the sky, measured in degrees north or south of the celestial equator; analogous to latitude on Earth. This Almanac gives the Sun’s declination at noon.

Eclipse, Lunar:
The full Moon enters the shadow of Earth, which cuts off all or part of the sunlight reflected off the Moon.
Total:
The Moon passes completely through the umbra (central dark part) of Earth’s shadow.
Partial:
Only part of the Moon passes through the umbra.
Penumbral:
The Moon passes through only the
penumbra
(area of partial darkness surrounding the umbra). See
Eclipses
[>]
for more information.

Eclipse, Solar:
Earth enters the shadow of the new Moon, which cuts off all or part of the Sun’s light.
Total:
Earth passes through the umbra (central dark part) of the Moon’s shadow, resulting in totality for observers within a narrow band on Earth.
Annular:
The Moon appears silhouetted against the Sun, with a ring of sunlight showing around it.
Partial:
The Moon blocks only part of the Sun.

Ecliptic:
The apparent annual path of the Sun around the celestial sphere. The plane of the ecliptic is tipped 23½° from the celestial equator.

Elongation:
The difference in degrees between the celestial longitudes of a planet and the Sun.
Greatest Elongation (Gr. Elong.):
The greatest apparent distance of a planet from the Sun, as seen from Earth.

Epact:
A number from 1 to 30 that indicates the Moon’s age on January 1 at Greenwich, England; used in calculations for determining the date of Easter.

Equinox:
When the Sun crosses the celestial equator. This event occurs two times each year:
Vernal
is around March 20 and Autumnal is around September 22.

Evening Star:
A planet that is above the western horizon at sunset and less than 180° east of the Sun in right ascension.

Golden Number:
A number in the 19-year cycle of the Moon, used in calculations for determining the date of Easter. (Approximately every 19 years, the Moon’s phases occur on the same dates.) Add 1 to any given year and divide by 19; the remainder is the Golden Number. If there is no remainder, use 19.

Greatest Illuminated Extent (Gr. Illum. Ext.):
When the maximum surface area of a planet is illuminated as seen from Earth.

Magnitude:
A measure of a celestial object’s brightness.
Apparent
magnitude measures the brightness of an object as seen from Earth.

Objects with an apparent magnitude of 6 or less are observable to the naked eye. The lower the magnitude, the greater the brightness. An object with a magnitude of −1, for example, is brighter than an object with a magnitude of +1.
Absolute
magnitude expresses how bright objects would appear if they were all the same distance (about 33 light-years) from Earth.

Midnight:
Astronomically, the time when the Sun is opposite its highest point in the sky. Both 12 hours before and after noon (so, technically, both
A.M.
and
P.M.
), midnight in civil time is usually treated as the beginning of the day, rather than the end. It is typically displayed as 12:00
A.M.
on 12-hour digital clocks. On a 24-hour time cycle, 00:00, rather than 24:00, usually indicates midnight.

Moon on Equator:
The Moon is on the celestial equator.

Moon Rides High/Runs Low:
The Moon is highest above or farthest below the celestial equator.

Moonrise/Moonset:
When the Moon rises above or sets below the horizon.

Moon’s Phases:
The changing appearance of the Moon, caused by the different angles at which it is illuminated by the Sun.
First Quarter:
Right half of the Moon is illuminated.
Full:
The Sun and the Moon are in opposition; the entire disk of the Moon is illuminated.
Last Quarter:
Left half of the Moon is illuminated.
New:
The Sun and the Moon are in conjunction; the Moon is darkened because it lines up between Earth and the Sun.

Moon’s Place, Astronomical:
The position of the Moon within the constellations on the celestial sphere.
Astrological:
The position of the Moon within the tropical zodiac, whose twelve 30° segments (signs) along the ecliptic were named more than 2,000 years ago after constellations within each area. Because of precession and other factors, the zodiac signs no longer match actual constellation positions.

Morning Star:
A planet that is above the eastern horizon at sunrise and less than 180° west of the Sun in right ascension.

Node:
Either of the two points where a celestial body’s orbit intersects the ecliptic.
Ascending:
When the body is moving from south to north of the ecliptic.
Descending:
When the body is moving from north to south of the ecliptic.

Opposition:
The Moon or a planet appears on the opposite side of the sky from the Sun (elongation 180°).

Perigee (Perig.):
The point in the Moon’s orbit that is closest to Earth.

Perihelion (Perih.):
The point in a planet’s orbit that is closest to the Sun.

Precession:
The slowly changing position of the stars and equinoxes in the sky caused by a slight wobble as Earth rotates around its axis.

Right Ascension (R.A.):
The celestial longitude of an object in the sky, measured eastward along the celestial equator in hours of time from the vernal equinox; analogous to longitude on Earth.

Solar Cycle:
In the Julian calendar, a period of 28 years, at the end of which the days of the month return to the same days of the week.

Solstice, Summer:
When the Sun reaches its greatest declination (23½°) north of the celestial equator, around June 21. Winter: When the Sun reaches its greatest declination (23½°) south of the celestial equator, around December 21.

Stationary (Stat.):
The brief period of apparent halted movement of a planet against the background of the stars shortly before it appears to move backward/westward (retrograde motion) or forward/eastward (direct motion).

Sun Fast/Slow:
When a sundial reading is ahead of (fast) or behind (slow) clock time.

Sunrise/Sunset:
The visible rising and setting of the upper edge of the Sun’s disk across the unobstructed horizon of an observer whose eyes are 15 feet above ground level.

Twilight:
For definitions of civil, nautical, and astronomical twilight, see
The Twilight Zone
[>]
.

1

The slowest-spinning object in the known universe is the nearest planet,
Venus
. A person could walk faster than it rotates.

2

The
density
of every neutron star is equivalent to what you would have after crushing a cruise ship until it’s the size of the ball in a ballpoint pen.

3

The longest star name still in common usage is Libra’s
Zubeneschamali.
The shortest is
Sun.

4

Mid-19th century scientists found that
sunspots
increase and then fade out in an 11-year cycle. Other scientists knew that earthly compasses strangely fluctuate in the same 11-year period. Yet it was years before anyone put these ideas together and realized that the Sun intimately affects our world through magnetism.

5

The large dark
blotches
on the Moon, called mares or seas, are all named for weather events (Ocean of Storms) or emotions (Sea of Tranquility).

6

There’s a separate “Earth” inside our planet:
Earth’s core
is not liquid iron as was once believed, but a solid ball the size of Pluto—and it spins faster than the rest of our world.

7

The Sun’s energy output is equivalent to the explosion of
91 billion 1-megaton
hydrogen bombs each second.

8

Astronomer Percival Lowell obsessively hunted for a ninth planet, “Planet X,” in vain. However, it was discovered from his observatory in 1930 by
Clyde Tombaugh,
and this is one reason that the name Pluto was chosen to honor Lowell. The first two letters are his initials.

9

The rocky celestial body with the shortest life span is Mars’s moon
Phobos
. The closest moon to any planet, it will crash into the Martian surface in 10 million years.

10

The first person who said that our planet moves was not Copernicus or Galileo. It was
Aristarchus of Samos.
This bearded genius was ignored for 18 centuries.

11

On a scale model in which Earth is a
dust mote
, the Sun would be one inch away and the size of the period at the end of this sentence. The nearest star would be another period 4¼ miles distant.

12

The most common object in the universe (possessing mass) is the
neutrino.
These tiny particles are more numerous than anything else by far. A trillion neutrinos fly through each of your fingernails every second.