Solar System Observer's Guide

Solar System Observer's Guide

by Peter Grego


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Product Details

ISBN-13: 9781554071326
Publisher: Firefly Books, Limited
Publication date: 03/04/2006
Pages: 256
Product dimensions: 5.00(w) x 7.70(h) x 0.60(d)

About the Author

Peter Grego is the editor of Popular Astronomy magazine and Director of the Lunar Section of the Society for Popular Astronomy. He also writes and illustrates a regular column in Astronomy Now and is the author of several books. He is a Fellow of the Royal Astronomical Society.

Read an Excerpt

1. Observing the Solar System

The Solar System, our cosmic backyard, comprises the Sun and everything within its gravitational domain. This includes all the major planets and their satellites, the asteroids and comets. Far and away the brightest and most easily accessible objects to the observer, the Sun and the Moon are spectacular when viewed through a small telescope (the Sun requires special precautions to view safely — see Chapter 15 for details on safe solar observing). Though they're much smaller and fainter, the brighter planets can also appear magnificent when viewed through the telescope eyepiece. Each planet displays its own unique set of phenomena. Spectacular phenomena visible in the Earth's atmosphere, such as aurorae and meteors, are best viewed with the unaided eye. Binoculars show bright comets well, often revealing considerable detail within their wispy tails.

Wherever you live, each clear night there's no shortage of opportunities to observe a variety of objects and phenomena within the Solar System, whether you're using the unaided eye, binoculars, or a telescope of any sort, small or large. As a whole, Solar System observation is far less hampered than deep-sky observation by the detrimental effects of light pollution, be it the type inflicted by thoughtless neighbors or the miasma blanketing whole urban regions.

Our cosmic backyard

In an outlying part of the Milky Way, around 25,000 light years from the Galactic center, lies a star called the Sun. It's a middling star, of average mass and brightness. There are hundreds of millions of stars just like it in the Milky Way. It takes the Sun around 226 million years to make one orbit around the Galactic center. Within a single Galactic orbit since the Sun's birth, the dusty, gassy remnants of the material surrounding the Sun flattened into a disk, and planets formed within it. Such a process is not unique, or even rare — disks of planet-forming material are thought to develop around most newborn stars. The biggest of these newly formed planets, Jupiter, failed to sweep up enough gas and dust for its core to become dense enough and hot enough to shine as a star in its own right. It did, however, accumulate enough material to become an imposing ball of gas one-tenth the diameter of the Sun. Three other large gas giants — Saturn, Uranus and Neptune — formed further away from the Sun. Nearer the Sun, four much smaller planets formed — Mercury, Venus, Earth and Mars — each having a solid, rocky surface. A curious thing happened on the third rock during the Sun's second orbit around the Galactic center — life appeared. Within the space of another dozen orbits of the Galactic center, an evolutionary branch of terrestrial life had developed into a race of sentient beings capable of viewing the Universe around them — from the Moon to the furthest reaches of the cosmos — with wonder and curiosity.

Although the Solar System, our cosmic backyard, is but a tiny place in the cosmic scheme of things, it is of the utmost significance to its inhabitants. The Earth is the only place we know where life exists, and the Sun is the only star that we can study in detail. Each major planet in the Solar System is unique. Each has its own individual surface conditions and its own peculiar type of atmosphere. While all of the solid worlds share a wide variety of geological processes, each possesses its own unique rocky make-up and its own set of unique surface features. The gas giants are all remarkably individual, too, each having a different atmospheric cocktail and each displaying a wide variety of colors and atmospheric activity.

Many moons

With the exception of Mercury and Venus, all the planets have satellites orbiting them. Most of them are pretty small in comparison with the planets they revolve around. Among Jupiter's more than 60 known satellites, four of them — Io, Europa, Ganymede and Callisto, known as the "Galilean moons" after their discoverer, Galileo — are exceptionally large, planet-sized objects. They are, in fact, big enough to be easily visible through a pair of binoculars, shining as bright star-like points of light near the dazzling Jovian disk. Yet these big satellites of Jupiter are tiny in comparison with Jupiter itself, and so are most other planetary satellites.

Earth's only natural satellite, the Moon, measures one-quarter the Earth's diameter, and is particularly large in comparison. Charon, Pluto's only known satellite, is comparatively even larger, and astronomers often refer to Pluto and Charon as a double planet. The Moon and Charon may share the same mode of origin, both having possibly been sliced off their respective parent planets in the remote past by the impact of another sizable planetary body. The Galilean moons, and the large satellites of Saturn, Uranus and Neptune, are likely to have been formed by gravitational accretion in orbit around their parent planets. Many of the smaller satellites in the Solar System, like diminutive Phobos and Deimos orbiting Mars, and the hundreds of irregular fragments orbiting the gas giants, are probably captured asteroids and cometary nuclei. Each of the four gas giants has a ring system made up of countless trillions of small orbiting moonlets ranging from grains of dust to house-sized chunks of rock and ice. Saturn's rings are the only ones visible through the eyepiece of a backyard telescope.

Interplanetary debris

There's much more to the Solar System than the major planets and their satellites. Judging by the rate at which new asteroids and comets are being discovered, there are likely to be millions of them larger than a kilometer in diameter in orbit around the Sun. Hundreds of thousands of asteroids (or "minor planets") orbit the Sun within the main asteroid belt, between the orbits of Mars and Jupiter. The asteroids are spread throughout such a large volume of space that if you were standing on one, the nearest one would likely appear as a distant, dim starlike point. In addition to the minor planets within the main asteroid belt, there are other distinct groups of asteroids elsewhere in the Solar System. Near-Earth asteroids are those whose orbits take them close to that of the Earth, including potentially hazardous asteroids — objects that could possibly collide with us at some point in the distant future. An interesting group called the Trojan asteroids are clustered at points far preceding and following Jupiter in its orbit around the Sun, locked there in a gravitational resonance.

It is thought that many of the asteroids in the main belt were part of larger bodies that broke up. These parent bodies, formed early in the history of the Solar System, grew hot inside and developed a core, mantle and crust. Most of the meteorites found on the Earth are fragments of asteroids, and by studying their composition we can tell what the original parent bodies were made of.

Comets frequently glide into the inner Solar System and brighten enough to be visible through binoculars or with the unaided eye. When far away from the Sun, chilling out in the interplanetary deep freeze, a comet is a pretty unimpressive sight — a solid ball of ices mixed with dirt and rock measuring just a few kilometers across. Known as the nucleus, this "dirty snowball" is made up of material left over from the formation of the Solar System. As it approaches the Sun, the comet's nucleus heats up and its ices sublimate, turning directly from a solid state into gas. Streaming off the nucleus, this gas carries with it grains of dust, forming a large coma shining by reflected and scattered sunlight, and it may go on to develop a prominent tail, perhaps spanning interplanetary scales.

Halley's Comet, the most famous cometary visitor, is just one of more than 160 periodic comets known to make regular visits to the inner Solar System. Halley's orbit takes it from the frigid realms beyond Neptune to the inner Solar System every 76 years. Some comets, such as the spectacular Hale-Bopp of 1997, have orb

Table of Contents

  1. Observing the Solar System
    • Our cosmic background
    • Planetary phenomena
    • Observing the planets
    • Lunar landscapes
    • Solar splendors
  2. The Solar System explorer's tools
    • Vision
    • Optical instruments
    • Binoculars
    • Telescopes
    • Eyepieces
    • Telescope mounts
    • Computers and astronomical programs
    • Recording the Solar System
  3. Mercury
    • A small rocky world
    • Orbit
    • Rotation
    • Apparitions
    • Transits
    • Mercury's phases and its appearance through the telescope
    • Recording Mercury
  4. Venus
    • An Earth-sized world
    • Orbit and rotation
    • Apparitions
    • Locating Venus in daylight
    • Telescopic observation
    • Venusian anomalies
    • Recording Venus
  5. Mars
    • Red planet
    • Martian atmospherics
    • Polar caps
    • Orbit and rotation
    • Apparitions and oppositions
    • Telescopic observation
    • A tour of Mars, westward from 0-360deg, in four sections
    • Recording Mars
    • The Martian moons
  6. Minor planets
    • Near-Earth asteroids
    • Far-out space rocks
    • Main-belt asteroids
    • Observing main-belt asteroids
    • Advanced studies
  7. Jupiter
    • Giant planet
    • Dynamic atmosphere
    • Jupiter's orbit
    • The Jovian system
    • Observing Jupiter
    • Recording Jupiter
    • A guide to Jovian activity
    • Galilean phenomena
    • Oppositions of Jupiter, 2006 to 2015
  8. Saturn
    • The ringed planet
    • Saturn's orbit
    • Majestic rings
    • Saturn's satellites
    • Observing Saturn
    • Observing the satellites
    • Recording Saturn
  9. Uranus
    • Herschel's planet
    • Rotation and orbit
    • Rings
    • Satellites
    • Observing Uranus
    • Recording Uranus
  10. Neptune
    • Orbit and rotation
    • Rings
    • Satellites
    • Observing Neptune
    • Recording Neptune
  11. Pluto
    • Distant planetary outpost
    • Orbit and rotation
    • Observing
    • Worlds beyond: Trans-Neptunian Objects
  12. Comets
    • Dirty snowballs
    • A vast, hidden cometary realm
    • Periodic comets
    • Cometary nomenclature
    • Great comets
    • Observing comets
    • Recording comets
  13. Meteors
    • Zodiacal light and the gegenschein
    • Annual meteor showers
    • Observing meteors
  14. Moon
    • Earth's natural satellite
    • The Moon's orbit
    • Libration
    • Lunar phases
    • Earthshine
    • A general tour of the
      Moon's surface
    • Observing the Moon
    • Recording the Moon
    • Eclipses
    • Transient Lunar Phenomena (TLP)
  15. Sun
    • Solar hub
    • Size, shape, axis and rotation
    • The Sun's ecliptic path
    • Seasons
    • Sun-Earth distance
    • Solar activity
    • White-light features
    • Limb darkening
    • Hydrogen-alpha features
    • Solar projection
    • Herschel wedge
    • Viewing the Sun through filters
    • Conditions for solar observing
    • Orientation of the solar disk
    • Counting sunspots
    • Recording the projected
    • Solar eclipses
  16. Aurorae
    • Observing aurorae
    • Recording aurorae


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