Astrophysical Directions by Michael Erlewine

 

 

In Astrophysical Directions
Introduction Coordinate Systems The Solar System The Solar Neighborhood The Galaxy Galactic Objects The Fixed Stars Star Clusters & Nebulae Non-Visual Astronomy External Galaxies Finder-Lists, etc.
The Solar Systems

 

  Planets

  The Sun

  The Moon

  Orbital Data

  Physical Data

  Planetary Satellites

  Invariable Plane of Solar System

  Solar Wind

  Asteroids

  Trojan Asteroids

  Meteors

  Famous Meteor Craters

  Comets

 

Planets

The Earth is a planet or secondary circling the Sun or primary at a mean distance of about 93 million miles. The mean Earth-Sun distance is taken as one astronomical unit (AU). The plane of the Earth's orbit around the Sun in the course of one year is called the plane of the ecliptic or zodiac. The ecliptic is the circle on the celestial sphere (at an infinite distance) at the intersection of the celestial sphere and the plane of the Earth's orbit. The Earth's axis of rotation is not perpendicular to the plane of the ecliptic, but inclined about 23.5 degrees to the perpendicular. The North Pole of the Earth does not point in the direction of the ecliptic north pole. (see Coordinate Systems)

There are nine planets, including the Earth, revolving around the Sun. The closest to the Sun is Mercury (at a mean distance of about .4 AU) and the most distant is Pluto (at a mean distance of 39.4 AU). The orbits of all the planets are quite close to the plane of the ecliptic except that of Pluto, which is inclined some 17 degrees.

Six of the nine planets have satellites. In addition, there are thousands of small bodies revolving around the Sun between the orbits of Mars and Jupiter, the asteroids or minor planets. The system, including the Sun, its planets, and asteroids, is referred to as the solar system. In addition to these main members of the solar system, there is a significant amount of gas, dust, and small solids (including meteors and comets), which may be collectively referred to as inter-planetary matter.

The main purpose of this section is to provide reference information useful in connection with sections. The following pages contain tables of pertinent information concerning the planets, their satellites, the asteroids, comets, and meteors. There are many good texts available describing the nature of our solar system in great detail and it is assumed that the reader either has some familiarity with our system or can obtain this at the local library. Here we are interested in the significant points and directions in space rather than an examination of the many different qualities of the members of our solar system.

Included is a diagram of our solar system out to and including Saturn. The outer or transcendental planets Uranus, Neptune, and Pluto are much too far out to fit on this paper. The other planets (pictured here) show the relative size and distance of the various orbits. Note the large asteroid belt between the orbits of Mars and Jupiter and the two groups of asteroids (Trojans) positioned roughly sixty degrees ahead and behind the giant planet Jupiter (see Asteroids for details).

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The Sun

The Sun is a variable star that is unlike any we know. It revolves east to west (in the direction of the signs of the Zodiac -- counterclockwise. The equator of the Sun is another fundamental reference plane to which we could refer all planetary motion. The inclination of the solar equator to the ecl iptic is 7°15' and the longitude of the ascending node to the ecliptic of 1950 is 75°04'. Some interesting data about our Sun:

  1. Period of synodic rotation 26.75 + 5.72sin in F d.
  2. Period of sidereal rotation (F = 17°)= 25.38 days
  3. Corresponding synodic period = 27.275 days
  4. Sun's angular velocity (F = 17°)= 2.865xl0-6 rad s-1
  5. Sun's radius = 864934.6 miles
  6. Sun's mass = 1.989(2)x 1033g
  7. Mean distance from Earth= 92.9558xl06 miles
  8. Mean equatorial horizontal parallax= 8.79418
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The Moon

Here is some basic data about the Moon:

  1. Mean distance from Earth = 384401 km
  2. Extreme range 356400406700 km
  3. Mean horizontal paralax 3422.60"
  4. Eccentricity of orbit = 0.0549
  5. Inclination of orbit to ecliptic = 5°08'43"
  6. Sidereal period (fixed stars)= 27.321661 ep. days
  7. Synodical month (New Moon to New Moon) = 29.5305882 ep. days
  8. Anomalistic month perigee to perigee) = 27.5545505 days
  9. Tropical month (equinox to equinox) = 27.321582 days
  10. Nodical month (node to node) = 27.212220 days
  11. Period of Moon's node (nutation, retrograde) = 18.61 tropical years
  12. Period of rotation of Moon's perigee (direct) = 8.85 years
  13. Moon's sidereal mean daily motion = 13°.176358
  14. Mean Transit interval = 24h 50.47m

 

Main periodic terms in the Moon's motion:

  1. Principal elliptic term in longitude 22639"sin g
  2. Principal elliptic term in latitude 18461"sin u
  3. Evection = 4586"sin (2D-g)
  4. Variation = 2370"sin 2D
  5. Annual inequality = -669"sin g'
  6. Parallactic inequality = -125"sin D, where g = Moon's mean anomaly, g' = Sun's mean anomaly, D = Moon's age and u= distance of mean Moon from ascending node.
  7. Inclination of lunar equator to ecliptic = 1°32.5'
  8. Inclination of lunar equator to orbit = 6°41'
  9. Mean Moon radius = 1738.2 km
  10. Moon mass = 1/81.301 mass of Earth
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Orbital Data

Orbital Data
Click image for a biger view

 

Legend for Orbital Data
(1) Planet, (2) Semi-major axis of orbit in astronomical units (AU) and kilometers, (3) Sidereal Period in Tropical years & days, (4) Synodic period in Tropical days, (5) Mean daily motion, (6) Mean orbital velocity in kilometers, (7) Eccentricity of orbit, (8) Inclination of orbit to Ecliptic Plane, (9) Ascending node to Ecliptic, (10) Mean longitute of Perihelion, [ "T" for 9 & 10 is century increment], (11) Longitude of planet for date given, (12) Perihelion: latest date & distance in AUs.

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Physical Data

Physical Data
Click image for a biger view

 

Legend for Physical Data
(1) planet, (2) semi-diameter (equator) at 1 AU & at conjunction or oposition, (3) Radius of Equator in kilometers & in relation to the Earth, (4) Ellipticity, (5) Volume in relation to Earth, (6) Reciprocal mass (includes Satellites), (7) Mass excluding Satellites, (8) Density, (9) Surface gravity: attractive and equator centrifugal, (10) Escape velocity, (11) Sidereal rotation (equatorial), (12) Inclination of equator to orbit, (13) Moment of inertia.

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Planetary Satellites

Planetary Satellites

Legend for Planetary Satellites
(1) planet, (2) semi-diameter (equator) at 1 AU & at conjunction or oposition, (3) Radius of Equator in kilometers & in relation to the Earth, (4) Ellipticity, (5) Volume in relation to Earth, (6) Reciprocal mass (includes Satellites), (7) Mass excluding Satellites, (8) Density, (9) Surface gravity: attractive and equator centrifugal, (10) Escape velocity, (11) Sidereal rotation (equatorial), (12) Inclination of equator to orbit, (13) Moment of inertia.

 

 

Click image for a biger view
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Invariable Plane of Solar System

The Invariable Plane of our solar system passes through the center of gravity of the system and is independent of the mutual perturbations of the planets. It is called 'invariable' because it remains unaltered, regardless of any and all motions within the planetary system. It is a plane through the center of mass, perpendicular to the orbital angular-momentum factor. This factor is made up of the angular momentum arising from orbital revolutions and from axial rotations.

As one planet decreases its eccentricity and inclination (over very long time periods), one or more orbits must at the same time be increasing their eccentricities and inclinations, whereby the total amount of eccentricity and inclination remains constant. Jupiter and Saturn largely determine the invariable plane, since they are the largest and heaviest of the planets. There has been some thought given to using the invariable plane as a fundamental reference plane on which to study planetary configurations. The center of mass of the solar system moves, with respect to the inertial system of reference, in a straight line with constant speed through space in a 250-million-year orbit or circle around the galactic center. The northern node of the invariable plane to the ecliptic is 107°03'46.99" in longitude (1950.0) with an inclination of 1°34'50" to the ecliptic plane. Thanks to Charles A. Jayne, Jr. for his research on this subject.

 

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Solar Wind

A high wind of hydrogen blows all night and day through our solar system. It emanates from the Sun and rushes past the Earth at some 400 km per second and out into interstellar space. It sweeps like a broom gases that have evaporated from planets and meteoritic dust. The solar wind is responsible for the outer portions of the Van Allen radiation belts, for the aurora in the Earth's atmosphere, and for terrestrial magnetic storms, perhaps even the general weather patterns.

A most important function of the solar wind, which acts like an aura out as far as Saturn, (during the years of high solar activity, the sunspot cycle) is to push back cosmic ray particles coming from outside our solar system. The intensity of cosmic rays reaching the Earth is cut in half during the years of highest solar activity.

One way of looking at this phenomenon is that the Earth and the inner planets are wrapped in a cloak or aura of Solar particles for several years and thus shielded from information trying to reach us from deep space. As the Sunspot cycle ebbs and the aura withdraws, the cosmic rays once again penetrate in greater numbers into the inner solar system and to the Earth. There were sunspot minimums in 1964 and 1976.

 

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Asteroids

The Asteroids (Greek, asteroides, "starlike"), also known as the minor planets or planetoids, constitute a group of bodies ranging from about 470 miles to a mile or two in diameter that revolve about the Sun in orbits that occur, in general, between those of Mars and Jupiter. It has long been known that the distance between Mars and Jupiter is proportionally larger than for any other two planets and Kepler even suggested that a planet might be found in this region of the solar system. The first asteroid was sighted in this region in 1801 (Ceres) and by 1807 three others were known (Pallas, Juno and Vesta). As of 1972 there were 1779 minor planets with determined orbits and an estimated 50,000 asteroids probably exist. The great majority of the asteroids move in orbits within a range of 2.1 to 3.5 astronomical units from the Sun and the orbital periods vary, in general, between 3.3 and 6 years, with a weighted average of 1.5 years. The orbits are somewhat more eccentric than those of the principal planets and the orbital planes are also more highly inclined to the plane of the ecliptic. The asteroids are more or less evenly spread between Mars and Jupiter, with " some exceptions. None has a period close to one-half, two fifths or one third of the orbital period of Jupiter and these spaces in the asteroid belt are termed the Kirkwood gaps.

It was first thought that these gaps were produced by perturbations caused by the giant planet Jupiter, but today it is felt that the disturbing actions of many asteroids on each other, in resonance, force them out of period. There is no precise information concerning the true mass or structure of any asteroid. Many astronomers believe that most are the broken fragments of two (or many) small planets that, formed between Mars and Jupiter, subsequently underwent violent collisions.

 

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Trojan Asteroids

One of the most interesting sub-groups of asteroids are the Trojans. In 1772, the French mathematician and astronomer, Lagrange considered the hypothetical case of a body of relatively small mass (such as an asteroid) revolving around the Sun in the same orbit as a heavy planet. He showed that if the Sun, the planet, and the asteroid were located at the corners of an equilateral triangle, the position of the asteroid with respect to the planet would remain essentially unchanged.

Such an asteroid was actually discovered in 1906 and subsequently a group of 15 or so turned up — the Trojan asteroids. The Trojans fall into two groups: one group of five asteroids precedes Jupiter in its orbit by 60 degrees of arc and the other ten follow it by that same angle. Today over 1,000 Trojans have been discovered and for some unknown reason there are at least twice many Trojans at the Lagrangian point ahead of Jupiter as there are behind it. Spectral studies show that, as a group, the Trojans are the darkest of all asteroids. They may be composed of debris left over after the formation of Jupiter or they may be accretions of interplanetary matter gravitationally attracted toward the giant planet.

A number of asteroids with highly inclined orbits also exist (one reaching within Mercury's orbit). Some of these cross the orbit of the Earth and some exhibit rotation. Asteroids are the subject of much attention at this time in astronomy.

 

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Meteors

Meteors or "shooting star" is a bright streak (meteor trail) across the night sky resulting from the heat generated when a particle or piece of matter traveling at a high velocity in space enters the Earth's atmosphere. The particle itself is called a meteor, but it is preferable to designate it as a meteoroid. A meteoroid then produces a meteor when it encounters and interacts with Earth's atmosphere. A very bright meteor is called a fireball, and a large fireball (particularly one accompanied by sparks and explosive noise) is called a bolide. While most meteoroids will disintegrate into small particles and dust upon entering our atmosphere, some of the very largest will make impact with the surface of the Earth creating large craters. A list of some of the most famous meteor craters follows this article.

There are two main types of meteors: sporadic and recurrent meteors (showers). Sporadic meteors may be seen on almost any night of the year at a rate of 5 to 7 per hour and show no preferred direction in the sky. The greatest frequency of sporadic meteors occurs after, rather than before, midnight. Between midnight and dawn an observer is facing the same direction as the Earth is moving in its orbit and he can see all of the meteors formed by the meteoroids traveling toward him (from the left), no matter what their velocity). On the other hand, between dusk and midnight, the only meteors that are visible are those produced by meteoroids coming toward him (from the right) with sufficient velocity to overtake the Earth.

The other type of meteor that occurs is the meteor shower. Meteor showers occur at relatively fixed times of year and seem to originate from a fixed point in the heavens known as a radiant. Meteor showers take their names from the constellation or star near where their radiant position is located and most occur each year with great regularity. The display of the Leonid shower on November 12, 1833 was so striking that meteors were described as "falling like snowflakes from the sky" and no section of the heavens was not filled with thousands of meteors. These permanent showers occur as the Earth sweeps through the concentrations of dust and debris in space. This debris is moving in orbit about the Sun. After A few days, the Earth moves through and beyond the particular debris. Orbits of a general sort are known for the principal showers and some of the major showers are presented in Figure A below. Most meteor shows occur regularly each year, some every few years, and in several cases a shower has been completely lost or has vanished.

Radio-echo technique has greatly expanded our understanding of meteor showers by allowing us to very accurately record these events. In at least three major cases, this technique has discovered new radiant points occurring only through the daylight hours (daytime showers).

 

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Famous Meteor Craters

Number Name / Location Year
Discovered
Lat. Long. # Largest Crater
Diameter
Rim Height
°    ' °    ' M Outer Floor
01 Barrington, Arizona, USA 1891 35N02 111W01 1 1240 39 190 m
02 Tunguska, Siberia, USSR 1908 60N55 101E57 10+ 52    
03 Odessa, USA 1921 31N48 102W30 2 170 3 m 4
04 Dalgaranga, Australia 1923 27S45 117E05 1 70   5
05 Osel, Kaalijarv, Estonia 1927 58N24 22E40 7 100   15
06 Campo Del Cielo, Argentine   28S40 61W40 many 75 1  
07 Henbury, Australia 1931 24S34 133E10 13 150   15
08 Wabar, Arabia 1932 21N30 50E28 2 100   12
09 Haviland, Kansas, USA 1933 37N35 99W10 1 14   3
10 Boxhole, Australia 1937 22S37 135E12 1 175   15
11 Wolf Creek, Australia 1947 19S18 127E46 1 820 30 60
12 Herault, France 1950 42N32 3E08 6 230 0 50
13 Chubb, New Quebec, Canada 1950 61N17 73W40 1 3400 100 380
14 Aouelloul, Mauritiania 1950 20N17 12W42 1 300   20
15 Brent, Ontario, Canada 1951 46N04 78W29 1 3200   70
16 Murgab, Tadzhik, SSR 1952 38N05 76E16 2 80   15
17 Deep Bay, Sask, Canada 1956 56N24 103W00 1 13000   340
18 Reiskessel, Bavaria 1904 48N53 10E37 1 24000    
19 Clearwater Lakes, Quebec 1954 56N10 74W20 2 26000   30

 

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Comets

It is believed that Comets are massive chunks of loosely packed ices -- frozen gases. In addition to ordinary water, these include carbon dioxide (dry ice), methane, cyanogen, and ammonia. Comets move in highly elliptical orbits (in most cases) and spend the majority of their time in the frigid regions at the very edge of our solar system. Once every 10,000 years or so they come close to the Sun, rapidly cross the inner portion of their orbit, and then speed back out again to the depths of space. During this fleeting visit to the solar neighborhood, the comet encounters sunlight, which melts and evaporates some of the ices. The solar wind particles (ever flowing out from the Sun) catch this comet material and blow it out into a long luminous tail that may stretch millions of miles, always in a direction away from the Sun. It is believed that practically all comets belong to the solar system and no clear-cut evidence for a visitor from external space has been yet found. Upwards of 800 assages (more than 500 individual comets) have been observed with sufficient accuracy to provide reliable orbital data. Some 300 move in nearly parabolic or in hyperbolic orbits, while about 200 move in elliptic orbits of measurable period.

Bright and spectacular comets are rare, one appearing on the average of every ten years or so. According to one theory, "new comets" come close to the Sun for the first time when the gravitational action of passing stars perturbs their original orbits. The lifetimes of comets appear to be quite short, once their perihelion distance from the Sun are reduced to 1 A.U. or so. They begin to disintegrate and disappear. Each return of the comet results in a loss of mass until, in some cases, the comet may break into pieces and disintegrate.

 

Orbit of Comet

Very bright comets were seen during the 19th century in 1811, 1835, 1843, 1861, and 1882 and this century in 1910, 1957, 1962, and 1965. Comet designation represents the order of their discovery in a given year (1910a, 1910b, and so on) as temporary identification, along with the name of the discoverer or discoverers (not more than three names). Later, a permanent designation is decided upon that includes the year,, followed by a Roman numeral in the order of perihelion passage. Periodic comets often bear the names of their discoverers or occasionally of the individual who computes the orbit. The famous Halley's comet received its name because of Halley's important prediction of its return in 1759.

The head of a comet often appears as a stellar nucleus surrounded by a fuzzy coma, which may extend for more than 100,000 kilometers. Most comets appear or become visible somewhere between the orbits of Jupiter and Mars, become brilliant and spectacular in the approach to the Sun, and show a rapid decrease in brightness as they recede from the Sun. During their departure, few are observed beyond 3 A.U.. Comets have long been a sign or believed to be an indication of powerful events soon to occur on Earth.

 

 

© Copyright © 1997 Michael Erlewine

 

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