DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Julian year
Gregorian Date (midnight) Julian Date (UTC) Modified Julian Date (UTC)
Jan 1, 1900 2415020.5 15020
Jan 1, 1980 2444239.5 44239
Jan 1, 2000 2451544.5 51544
For more information, see:
http://www.capecod.net/ pbaum/date/back.htm
(and http://tycho.usno.navy.mil/systime.html)
Julian year See year.
Juliet Moon of Uranus also designated UXI.
Discovered by Voyager 2 in 1986, it is a small,
irregular body, approximately 42 km in radius.
Its orbit has an eccentricity of 0.0005, an inclination
of 0 ◦ , a precession of 223 ◦ y −1 , and a
semimajor axis of 6.44 × 10 4 km. Its surface is
very dark, with a geometric albedo of less than
0.1. Its mass has not been measured. It orbits
Uranus once every 0.493 Earth days.
Junge particle distribution A power-law
distribution function [particles m −3 µm −1 ] often
used to describe the particle concentration
[particles m −3 ] per unit size interval [µm] vs.
equivalent spherical diameter of particles; the
number density for particles of equivalent diameter
x is proportional to x −k . In natural waters
k is typically 3 to 5.
Juno Third asteroid to be discovered, in
1804. Dimensions 230 by 288 km. Orbit: semimajor
axis 2.6679 AU, eccentricity 0.25848, inclination
to the ecliptic 12 ◦ .96756, period 4.358
years.
Jupiter A gas giant, the fifth planet from
the sun with orbital semimajor axis 5.20AU
(778,330,000 km), orbital eccentricity 0.0483;
its orbital period is 11.8623 years. Jupiter is the
largest planet, with equatorial radius 71,492 km,
and mass 1.9 × 10 27 kg; its mean density is
1.33 gm/cm 3 . It has a surface rotational period
of 9.841 hours. Jupiter has an orbital inclination
of 1.308 ◦ and an axial tilt of 3.12 ◦ . Jupiter has
an absolute magnitude of −2.7, making it usually
the fourth brightest object in the sky, after
the sun, moon, and Venus; occasionally Mars
exceeds Jupiter in brightness.
© 2001 by CRC Press LLC
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The model structure of Jupiter envisions a
rocky core of order 10 to 15 M⊕, at a temperature
≈ 2×10 5 K, maintained by the gravitational
compression of the planet. Because of this heating,
Jupiter radiates considerably more energy
than it receives from the sun. Above this core is
a layer of liquid metallic hydrogen (at pressure
depths exceeding 4 million atmospheres). Liquid
metallic hydrogen is an electrical conductor
and large scale currents in this material produce
Jupiter’s magnetic field, via the dynamo effect.
Jupiter’s field is 14 times stronger at Jupiter’s
“surface” equator (4.3 Gauss), than is Earth’s at
its equator (0.3 Gauss). Jupiter’s magnetic field
is roughly dipolar, with its axis offset by 10,000
km from the center of the planet and tipped 11 ◦
degrees from Jupiter’s rotation axis. Jupiter’s
magnetosphere extends more than 650 million
kilometers outward, but only a few million kilometers
sunward.
Above the metallic hydrogen is a region composed
primarily of ordinary molecular hydrogen
(90%) and helium (10%) which is liquid in the
interior and gaseous further out.
Three distinct cloud layers exist in the atmosphere,
consisting of ammonia ice, ammonium
hydrosulfide, and a mixture of ice and
water. Jupiter’s atmosphere has high velocity
winds (exceeding 600 km/h), confined in latitudinal
bands, with opposite directions in adjacent
bands. Slight chemical and temperature differences
between these bands are responsible for
the colored bands that dominate the planet’s appearance.
The reddish colors seem to be associated
with the highest clouds, the bluish colors
with the deepest. Intermediate clouds have
brown-cream-white colors.
Jupiter posseses a very long-lived atmospheric
phenomenon, the Great Red Spot, which
has been observed for over 300 years. This is
a reddish colored elliptical shaped storm, about
12,000 by 25,000 km.
Jupiter has three faint and small, low (about
0.05) albedo rings (called Halo, Main, and Gos-