- The solid surface layer of a planet. For the Earth, the crust varies from eight km thick
under the oceans to about 70 km thick below the continents. The crust of a planet encloses
- The ratio of the mass of an object to its volume.
- differential rotation
- The rotation of a body such that parts rotate at different speeds. Thus, parts
near each other at one time will later be seperated.
- The amount by which an orbit deviates from a perfect circle: e = c/a, where c
is the distance from the center to the focus and a is the semi-major axis. In the
Solar System, e = 1 - (q/a), where q is the perihelion
- The partial or total obscuration of a bright object by a shadow. A solar eclipse occurs
when the Moon passes between the Earth and the Sun, blocking out the light from the Sun. A
lunar eclipse occurs when a the Earth passes between the Sun and the Moon, and the Earth's
shadow obscures the full Moon.
- The plane of the Earth's orbit. As seen from the Earth, this is the apparent path of the
Sun on the celestial sphere.
- Sir Arthur Eddington (1882 - 1944) was a British astronomer famous for his work in
Galactic dynamics, stellar interiors and the astrophysical implications of Einstein's theory of relativity. He spent most of his career at
Cambridge University, where he wrote his most important professional books: Stellar
Movements and the Structure of the Universe and The Internal Constitution of the
Stars. In the former, he initiated a new field of stellar dynamics; in the later, he
established the bases of the study of stellar interiors. These included the transport of
energy by radiation; the equilibrium of gravitation, gas pressure, and radiative pressure;
and culminated in the mass-luminosity law for stars.
By leading an expedition to West Africa in 1919 to observe the shift in apparent positions
of stars near the limb of the Sun during a total solar eclipse, he verified one of the
three predictions of the theory of general relativity. He wrote several popular-level
books, such as The Nature of the Physical World, in which he explains the tenets of
modern physics in a way which is still very readable. His greatest scientific error was
probably his strong disagreement with S.
Chandrasekhar over the theoretical upper limit on the masses of white dwarf stars (1.4
solar masses) on the basis that the stars could not know that, since the future degenerate
nature of their material would not permit a higher mass, they would have to shed mass. In
his later life, Eddington attempted to derive many of the fundamental constants of nature
(Hubble constant, relative strengths of the gravitational and electrical forces, the
velocity of light, etc.,) from a minimum set of constants. For a recent biography, see Eddington:
The Most Distinguished Astrophysicist of His Time by S. Chandrasekhar (Cambridge
University Press, 1983)
- Albert Einstein (1879 - 1955) was a German-born, naturalized-American theoretical
physicist who is famous for his theories of special and general relativity which related
space, time, and motion. He received the Nobel Prize in physics in 1921 for his
explanation of the photoelectric effect. In his most productive year in physics, 1905, he
published his papers on the photoelectric effect, Brownian motion, and special relativity
while discharging his duties as a Swiss Patent examiner in Berne. He is the author of the
most famous equation, E=mc2, which relates the quantities of mass and energy.
The theories of relativity explain the effects of the postulate that the speed of light
appears to be the same when observed from any inertial reference frame, no matter what the
speed of the observer. He is considered the last of the three giants of the science of
mechanics: Galileo, Newton, and Einstein.
- Electrons are small nuclear particles with a negative electrical charge usually found
moving around atomic nuclei. When not associated with atomic nuclei, they are often
referred to as beta particles. They have a mass of 9.10 X 10-28 grams;
this is 1/1836 the mass of a proton.
- The difference in the celestial longitudes of the Sun and a planet; the Sun-Earth-planet
angle. Eastern elongations appear east of the Sun and are visible in the evening;
correspondingly, western elongations of planets occurs when they are visible in the
morning. An elongation of zero degrees is called conjunction (aligned with the Sun); one
of 180° is called opposition (planet is overhead at midnight); and one of 90° is called
quadrature (overhead at sunrise or sunset).
- escape velocity
- The minimum velocity that a body requires to permanently escape from a primary body. The
escape velocity for the Earth is 11.2 km/sec and is the same for a molecule of hydrogen or
a space ship. The escape velocity from the Sun is 618 km/sec; to escape from the Solar
System at 1 AU, one needs a velocity of 49 km/sec. Mathematically, the velocity of escape
is given by
- Ve = (2GM/R)1/2
- where G is the universal gravitational constant, M is the mass of the parent body, and R
is the distance between the parent and escaping body.
- Galileo Galilei (1564 - 1642) was an Italian astronomer and physicist who founded the
modern scientific method of deducing scientific laws from observations and experiments. He
also proclaimed mathematics the language of science. He was the first modern scientist. In
physics, he was the founder of the mechanics, the science of the position, motion, and
dynamics of physical bodies. He developed the law of the pendulum, the concept of inertia,
and invented the thermometer. His initial fame came as the first scientist to use a
telescope to examine the heavens. He discovered the terrestrial character of the Moon,
sunspots, and the four major satellites of Jupiter (now called the Galilean satellites).
He challenged the Catholic Church over the issue of scientific theory vs truth concerning
the heliocentric theory. He lost. He is considered the first of the three giants of the
science of mechanics: Galileo, Newton, and Einstein.
For an excellent description of Galileo's conflict with the Catholic Church, see The
Crime of Galileo by Giorgio de Santillana (University of Chicago Press, 1955).
- gas giant
- A planet sufficiently massive that hydrogen cannot escape at ambient temperatures. Such
planets have very thick atmospheres that become increasingly dense at lower altitudes.
Most of a gas giant planet is thought to be liquid hydrogen surrounding a small,
rocky core. The gas giants of our Solar System are Jupiter, Saturn, Uranus, and
Neptune; they also are called the jovian planets.
- A measure of magnetic field strength: 10-4 x newton / (coulomb/sec)(meter) .
The magnetic field of the Earth, near the Earth's surface, has a strength of about
one gauss. This unit is named in honor the famous German astronomer, mathematician,
and physicist Karl Friedrich Gauss (1777 - 1855). [While he may be remembered by
many as a mathematician first, we note that while he was merely a Professor of Mathematics
at Göttingen, he was the Director of the Göttingen Observatory.] He invented the
method of least squares (which you'll learn if you go on in the physical sciences).
When the first asteriod, Ceres, was discovered by Guiseppe Piazzi, only three measurements
of its position were made before it was lost again. Gauss quickly developed a
technique for determining the orbit a a Solar System body from only three observations and
the asteroid was recovered at its predicted position. Unfortunately, much of
Gauss's mathematical work went unpublished and had to be rediscovered by others. He
is honored in magnetic work because he devised a logical set of units for magnetic
- A fairly common, but very beautiful, atmospheric affect. It is rings of color, usually 5
or 10 degrees across, seen at the antisolar point. They are usually seen against a white
background. They are most easily seen from aircraft surrounding the shadow of the aircraft
on the top layer of clouds. They can also be seen against the fog with a bright sun behind
you. Like rainbows, the colors are caused by the water droplets in the air but the
details, such as the characteristics of the polarization of the light, show that it is a
different, and still unexplained, phenomenon.
- gravitational lens
- The apparent deflection of light as it passes by a massive object. This is observed when
the light from a distant object passes through the gravitational field of an intervening
object on the way to the observer. Analogous to the effects of an optical lens, the image
of the more distant object may be shifted, distorted into arcs, or broken into two or more
images of the background object. When the deflecting object is moving, the deflected image
may appear to brighten symmetrically and achromatically. It was Newton
who first suggested, in his book Optics(1704), that the action of bodies should
bend the rays of light. There is indeed a predicted deflection of light from classical
Newtonian mechanics (0.85 arcseconds for a ray of light grazing the surface of the Sun)
but the full equations of Einstein's theory of general relativity
predict twice that deflection. Current measurements confirm the predictions of general
relativity to better than 1%. Arthur Eddington was the first to
predict that multiple images were possible. In 1937, Fritz Zwicky predicted that multiple
images should be observable and noted that the amplification by lenses would permit the
study of objects at great distances.
- green flash
- A rare but lovely optical affect in the atmosphere at sunset. As the sun sets, usually
over the ocean, there may be a brief moment when the sky around the area of the sunset
turns green. This is thought to be because the red light from the sun is blocked by the
limb of the Earth (the sun just having set) and the blue light is very scattered by the
atmosphere because the light is passing through a much longer path length than when it is
overhead. The only remaining light is green..
- greenhouse effect
- An imbalance of heat into a planet compared to the heat radiated. Different albedos and emissivities of gases in the planet's atmosphere cause the
radiation entering the atmosphere to be converted to infrared wavelengths to which the
atmosphere is opaque, preventing re-radiation. The trapped radiation heats the
atmosphere and the planet's surface.
- heavy bombardment era
- A time early in the history of the Solar System when the planets and major satellites
were rapidly bombarded with many meteors. This often caused melting of the crusts of the
target bodies and ended about 3.9 billion years ago.
- Hertz is a unit of frequency; cycles or vibrations per second. Very few people are so
lucky as to have their names, in lower case(!), become the nomenclature for a very common
measure. Heinrich Rudolf Hertz (1857 - 1894) lived only 36 years, dying of blood
poisoning. He realized, in 1888, that Maxwell's equations implied that electric
waves could travel through air. He experimentally proved this prediction by
generating and detecting what are now called radio waves.
- The property of an object to resist acceleration. As one of the most basic
concepts of physics, it is still poorly understood. It is believed to be a property
of bodies primarily due to their interaction with masses at the edges of the Universe.
Momentum (mass times velocity) is a measure of inertia and changes in momentum,
which must be due to a force, permit a measure of inertial mass.
- inferior planet
- A planet with a smaller orbit than the Earth's orbit; the opposite of superior planet.
- An atom that has become electrically charged by the addition or loss of one or more
- The region of a planetary atmosphere with a sufficient free electron density to affect
radio communication. Very little ionization is required: it varies between one per
thousand molecules to one per 100 million molecules in different regions. The bottom
of the ionosphere in the Earth's atmosphere is at an altitude of about 80 kilometers
during the day, rising to about 100 kilometers during the night. The ionosphere
starts at the top of the mesosphere (beginning of
the thermosphere) and continues to about 1000
- A temperature scale that uses absolute zero (-273ºC or -460ºF) as its starting point. Absolute zero is when there is no longer any thermal (kinetic) energy. The scale units are the same as celsius, so a change of 1ºC is equal to a change in 1 Kelvin. Notice that Kelvins do not use the degree ( º ) symbol.
- Johannes Kepler (1571 - 1630) was a German astronomer who used Tycho Brahe's
observations of Mars to discover three laws (Kepler's Laws) describing planetary motion.
These laws provide the bridge between the aesthetically-driven theory of Copernicus and the physics-driven mechanics of Galileo
and Newton. He also wrote the first science fiction story, Solemnium,
about a man who travels to the Moon.
- A thousand hertz: a thousand cycles per second.
- lunar eclipse
- Eclipse of the Moon caused by the shadow of the Earth falling on
the disk of the full Moon when the the Sun, Earth, and Moon are aligned. Unlike total solar eclipses, which are only visible along a narrow path for a
few minutes, lunar eclipses are visible from all of the nighttime globe and last for over
- magnetic field
- The region of space near a magnetized body within which magnetic forces can be detected.
- magnetic field lines
- A representation of the characteristics of a magnetic field.
These lines are drawn tangent to the magnetic force and the spacing of the lines is closer
where the magnetic field strength is greater.
- The area immediately above the atmosphere composed of charged particles trapped in the magnetic field of the planet. The particles spiral around
the magnetic field lines, eventually reflecting back when the magnetic field lines
converge near the magnetic poles of the planet. If they penetrate too deeply into
the ionosphere before reflecting back, they may interact with
atoms of the upper atmosphere, creating beautiful aurora. The
magnetosphere acts as an excellent shield against the damaging, high speed charged
particles from space.
- A measure of the amount of light received from a celestial object. Based on ancient
Greek classifications, objects with smaller magnitudes are brighter ("those objects
of the first magnitude" referred to the brightest objects). Since it originated in
the appearance of objects perceived by the human eye, it is a scale of ratios rather than
a linear scale: an object one magnitude brighter than another is about 2.512 times
brighter. The basis is a difference of five magnitudes is a difference in brightness of
100; thus, a difference of one magnitude is the fifth root of 100. The difference in
magnitudes between two stars of brightnesses l1 and l2 is
- m1 - m2 = -2.5 log(l1 / l2).
- The brightest stars are about magnitude 0, the faintest stars
visible to the unaided are about magnitude 5.
- major axis
- The maximum diameter of an ellipse.
- The layer of a planet between the core and the crust. In the Earth, the mantle is about
2900 km thick and is composed rocks of iron and magnesium combined with silicon and
- A measure of the total amount of material in a body. This is determined by either
its inertial properties or by its gravitational interaction with
- A million hertz; a million cycles per second.
- The region in a planetary atmosphere of the first temperature maximum. It lies
above the stratosphere and below the first
major temperature minimum (mesopause) found in the Earth's atmosphere at an altitude of
about 80 kilometers. This is the region where most meteors disappear and
temperatures are comparable to those at the Earth's surface. The mesosphere is in
radiative equilibrium between the ultraviolet ozone heating of the upper ozone region and
infrared ozone and carbon dioxide cooling by radiation into space.
- The incandescent streak of light seen when a meteorite passes through the atmosphere of
a planet. Often referred to as a falling star.
- A particle of solid interplanetary matter that has fallen to the surface of a planet.
- A small sized piece of solid matter in interplanetary space. When it stikes a planet, it
becomes a meteorite.
- minor planet
- A small, rocky planet-like body of the Solar System; also known as an asteroid. They range in size from boulder-size
to a few hundred miles across. They are usually irregular in shape and occupy the
asteroid belt region between Mars and Jupiter at about 2.8 AU
(see the discussion of the Titius-Bode
law) . They typically have mildly eccentric
orbits (e = 0.1 to 0.3) but a few are sufficiently eccentric that they pass by the orbit
of the Earth.
- Neutrons are heavy nuclear particles with no electrical charge and approximately the
same mass as protons. With protons, they are the principal
components of atomic nuclei. They are composed of two "down" quarks and
one "up" quark.
- Issac Newton (1642 - 1727) was an English physicist and mathematician who is regarded as
probably the greatest physicist to have ever lived. His fame centers on his description of
inertia, mechanics, and the law of gravitation which he published, under the prodding of
English astronomer Edmund Halley, in Mathematical Principles of Natural Philosophy.
In order to complete his studies of mechanics, he invented calculus. Oddly, he spent most
of his time in the pursuit of alchemy (an early specialty of chemistry devoted to turning
ordinary metals into gold) and his study of the Bible. His final major work was on optics
including the description of the composition of white light from the colors of the
spectrum, the Newtonian reflecting telescope (still used today by amateur astronomers),
and the corpuscle theory of light. He is considered the second of the three giants of the
science of mechanics: Galileo, Newton, and Einstein.
- nuclear fission
- The breakup of an atomic nucleus into two or more lighter nuclei. This occurs
naturally during radioactive decay and is the source of energy for fission (atomic) bombs.
See nuclear fusion.
- nuclear fusion
- The forming of a heavier atomic nucleus by joining lighter nuclei. After the fusion, the
resulting nucleus has a slightly lower mass than the sum of the masses of the nuclei that
formed it. That mass deficit appears in the form of energy according to the famous
equation E = mc², where E is energy, m is mass, and c
is the speed of light. The amount of energy released is enormous and is the primary
source of energy in the center of stars. It is also the source of energy for fusion
(hydrogen) bombs, which can use the much less powerful fission (atomic) bombs as a detonator.
- The amount by which a spheriod is flattened at its poles. Mathematically, it is formed
by rotating an ellipse about its minor axis. Astronomically, large bodies such as
planets and stars are oblate because of their rotation.
- The obliquity of the ecliptic is the angle between the planes of the celestial equator and the ecliptic.
It is equal to the tilt of the Earth's axis: about 23½°.
- The point on an orbit of a body revolving around the Sun where it is closest to the
center of the Sun. The perihelion of the orbit of the Earth occurs in January. Why is it
so cold in Cleveland in January if the Earth is closest to the Sun then?
- The closest point to the Earth in an orbit around the Earth. Usually refers to the
orbit of the Moon or of an artificial satellite.
- A photon is a quantum of energy. Photons make up all electromagnetic radiation, which includes the light we see, radio waves, x-rays, etc. The amount of energy carried by a photon is directly related to the frequency of the electromagnetic wave it represents: E = hf, where h is Planck's Constant and f is the frequency of the E&M wave.
- The visible surface of the sun, just above the upper-most layer of the interior and below the chromosphere.
- Protons are heavy nuclear particles carrying a positive electrical charge. Along
with neutrons, they are the principal components of atomic
nuclei. It has a mass of 1.67 x 10-24 grams, 1836 times the mass of an electron. Protons are composed of two "up" quarks and
one "down" quark.
- A measure of angle; there are 2p radians in a circle. The arc
subtended by a radian is equal to the radius of the associated circle.
- The transmission of energy through electromagnetic radiation; a method of transmitting
energy through a vacuum. Radiation is composed of particles of energy called
photons. The other two methods of transfering energy are convection and conduction.
- Refractory materials form solids at very high temperatures. The opposite of volatile. Tungsten is a refractory material.
- relative humidity
- The percentage of water in an atmosphere relative to the maximum possible at that
temperature and pressure.
- Opposite direction as the prevailing direction.
- retrograde motion
- An apparent westward motion of a planet on the celestial sphere with respect to the
stars. This is the opposite of the usual eastward motion and is used as a proof of the
motion of the Earth about the Sun.
- retrograde rotation
- Rotation in the opposite direction as the typical rotation in the system. Among the
planets, for example, Venus has retrograde rotation; that is, it rotates from east to west
rather than west to east like the other planets. Its rotation, clockwise as viewed from
the north, is the opposite as the direction of revolution of the planets about the Sun.
- semimajor axis
- The semimajor axis of an ellipse is half of the maximum diameter.
- shepherd satellite
- Small satellites found on the margins of planetary rings that, through gravitational
perturbations, confine the rings to the inside or outside of their orbits. For
example, the Saturnian shepherds Prometheus and Pandora bracket the F ring and keep the
ring particles confined to a narrow, sharp-edged ring. A rarity in science, shepherd
satelittes were predicted shortly before they were discovered.
- sidereal period
- The time interval between two successive alignments of a celestial object with respect
to the stars. Compare to synodic
- solar eclipse
- The total or partial obscuration of the Sun by the Moon. A solar eclipse occurs at a
given point on the surface of the Earth once every 360 years, on average. Each year at
least two and no more than five solar eclipses occur. The Moon's shadow moves east across
the surface of the Earth at about 3700 km per hour. Solar eclipses can be partial, annular, or total. Total solar eclipses are spectacular sights
and are worth the effort usually required to journey to a site covered by their narrow
path. Totality cannot last for more than about seven minutes. Only during a total
solar eclipse is the remarkable solar corona visible.
- solar wind
- The outflow of charged particles from the solar corona into space.
Because of the high temperature of the particles (mostly protons and electrons) of
the corona, they are moving at speeds higher than the solar escape
velocity. At the orbit of the Earth, these particles are moving at about 500
km/sec. Some of these particles are captured by the magnetic fields of the.planets,
forming their magnetospheres.
- Intensity or observed flux of electromagnetic radiation arranged as a function of
wavelength. A rainbow is an example of a spectrum of visible sunlight dispersed by the
prismatic affect of raindrops. The plural of spectrum is spectra.
- Sputtering is the impact of high speed atomic particles on surfaces of materials.
These impacts can cause chemical changes on the target material and/or knock components of
the surface free.
- Upper stratosphere.
- The region of a planetary atmosphere immediately above the troposphere and having an approximately constant temperature.
In the Earth, the stratosphere is thickest at the poles and thinner or even
nonexistant over the equator. The area of maximum atmospheric ozone, essential to
blocking ultraviolet radiation from the Sun, and rare nacreous clouds are found in the
stratopause at an altitude of about 25 kilometers. Temperatures are similar to
artic winter temperatures.
- superior planet
- A planet with a larger orbit than the Earth's orbit; the opposite of inferior planet.
- Time interval between two successive alignments of a celestial object with respect to
the Sun as seen from the Earth. The synodic period of Jupiter is the time it takes
Jupiter to return to a given location in the sky (e.g., on the meridian at
midnight) as seen from the Earth. Mathematically,
- (Synodic period)-1 = (Earth's period)-1 -
(Sidereal period) -1 for superior planets; or,
- (Synodic period)-1 = (Sidereal period) -1 - (Earth period)-1
for inferior planets.
- synodic month
- The period of time between two successive identical phases of the moon; this is the
lunar month.. 29.53 days
- terrestrial planets
- Planets, like the Earth, with solid crusts and thin, shallow
atmospheres. In our solar system, they are Mercury, Venus, Earth, and Mars.
- The region in planetary atmospheres above the major temperature minimum, in the mesosphere, to outer space. In the
Earth's atmosphere, it starts at an altitude of about 80 kilometers and has molecular
temperatures that rise into the thousands of degrees. This is the domain of
aurorae. The molecular densities are so low that the gas is unable to cool through
infrared radiative cooling to space due to the infrequency of molecular (in
particular, carbon dioxide) cooling.
- tidal force
- A differential gravitational force that tends to deform a body.
- Tombaugh, Clyde
- Clyde William Tombaugh (1905 - 1997) was the last astronomer to discover a new planet in
our solar system. In 1928, Clyde submitted observations that he had made as an amateur
astronomer with a homemade 8-inch telescope to Lowell Observatory in Flagstaff, AZ. He was
hired to search for the ninth planet based on predictions by Percival Lowell. He found
Pluto in 1930, early in the search process. It soon became clear that Pluto was too small
and faint to have exerted the gravitational forces to have perturbed Neptune's motion. He
spent much of his career at New Mexico State, teaching astronomy and geology. At a MIRA
lecture, he related how someone had asked him for his original 8-inch telescope for a
museum exhibit. The octogenarian responded,"sure, as soon as I'm done with it".
- The upper region of the troposphere. In the
Earth's atmosphere, the tropopause occurs between altitudes of about 6 to 18 kilometers
and is the domain of high winds and highest cirrus clouds.
- The region of a planetary atmosphere nearest the surface, having a more or less uniform
decrease of temperature with altitude. In the Earth's atmosphere the nominal rate of
temperature decrease is 6.5°K/km but inversions are common. The troposphere, the
domain of weather, is in convective equillibrium with the sun-warmed surface of the Earth.
- Materials that are realtively easily vaporized. Accordingly, such materials solidify at
very low temperatures. This is the opposite of refractory.
The version you are examining is: 1.15 (last updated: August 9 , 1998 by bw)
MIRA's Field Trips to the Stars © 1995-1998
Monterey Institute for Research in Astronomy