Pariksha SaathiLesson 1 of 10 · Physical and World Geography
The Universe and the Solar System: Galaxies, the Sun, Planets, Moons and Eclipses
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The universe and galaxies
The Big Bang theory, proposed by Georges Lemaitre in 1927 and supported by Edwin Hubble's discovery in 1929 that galaxies are moving apart, holds that the universe began about 13.8 billion years ago and is still expanding. A galaxy is a vast system of stars, gas and dust held together by gravity. Our galaxy, the Milky Way (Akashganga), is a barred spiral, and the solar system lies in one of its arms; the nearest large galaxy is Andromeda. A light year, the distance light travels in a year, is about 9.46 trillion km. After the Sun, the nearest star is Proxima Centauri, about 4.2 light years away.
The Sun
The Sun is a medium-sized star about 4.6 billion years old, holding about 99.8 per cent of the solar system's mass. It is mostly hydrogen and helium and shines by nuclear fusion of hydrogen into helium in its core, at about 15 million degrees Celsius. Its visible surface is the photosphere (about 5,500 to 6,000 degrees Celsius), above which lie the chromosphere and the corona, seen in a total solar eclipse. Sunspots are cooler, darker patches that follow a cycle of about 11 years. The Sun is about 150 million km away, a distance called one astronomical unit, and its light reaches us in about 8 minutes 20 seconds.
Inner planets
Mercury, Venus, Earth and Mars are small, rocky terrestrial planets. Mercury is the closest and smallest, with no moon, almost no atmosphere and the shortest year, 88 days. Venus is the hottest planet, because its thick carbon dioxide atmosphere traps heat, and the brightest after the Moon, seen as the morning or evening star. Called Earth's twin for its size, it spins slowly and backwards, so one spin (243 Earth days) takes longer than one orbit (225 days), and the Sun rises in the west. Earth is the densest planet and the only one known to have life. Mars, the Red Planet, owes its colour to iron oxide; it has two moons, Phobos and Deimos, and Olympus Mons, the highest volcano in the solar system. India's Mangalyaan entered Mars orbit on 24 September 2014, making India the first country to succeed at its first attempt.
Outer planets
Jupiter, Saturn, Uranus and Neptune are giants of gas and ice, and all four have rings. Jupiter is the largest and spins fastest, a day of about 10 hours; its Great Red Spot is a giant storm. Galileo discovered its four large moons, Io, Europa, Ganymede and Callisto, in 1610, and Ganymede is the largest moon in the solar system, bigger than Mercury. Saturn, with the grandest rings, is the least dense planet, lighter than water; its moon Titan is the only moon with a thick atmosphere. Uranus, found by William Herschel in 1781, the first planet discovered with a telescope, spins on its side (axis tilted about 98 degrees) and, like Venus, rotates from east to west. Neptune, the farthest, takes about 165 years to circle the Sun; it was found in 1846 where calculations had predicted, and its moon Triton orbits backwards.
Dwarf planets and small bodies
In August 2006 the International Astronomical Union (IAU) at Prague defined a planet as a body that orbits the Sun, is nearly round and has cleared its orbit of other objects. Pluto fails the last test and became a dwarf planet; the IAU recognises five: Ceres, Pluto, Eris, Haumea and Makemake. Ceres is the largest body in the asteroid belt between Mars and Jupiter, and Pluto, whose largest moon is Charon, lies in the Kuiper Belt beyond Neptune. Comets are lumps of ice and dust whose tails always point away from the Sun; Halley's Comet returns about every 76 years, was last seen in 1986 and is due in 2061. A meteor (shooting star) burns up in the air; one that reaches the ground is a meteorite, and Lonar Lake in Maharashtra fills a meteorite crater.
The Moon
The Moon, Earth's only natural satellite, is about 384,400 km away, and its gravity is about one-sixth of the Earth's. It takes about 27.3 days both to orbit the Earth and to spin once, so the same side always faces us; one cycle of phases takes about 29.5 days. Neil Armstrong of Apollo 11 first walked on it in July 1969, and India's Chandrayaan-1 (2008) helped detect water molecules on its surface.
Eclipses
A solar eclipse occurs on a new moon day (amavasya), when the Moon passes between the Sun and the Earth. It is total when the Moon hides the whole Sun and annular when the Moon, being farther away, looks too small and leaves a bright ring of the Sun showing. A lunar eclipse occurs on a full moon night (purnima), when the Earth comes between the Sun and the Moon; in a total lunar eclipse the Moon looks coppery red because the Earth's atmosphere bends red light onto it. Eclipses do not occur every month because the Moon's orbit is tilted about 5 degrees to the Earth's, so the three line up only at certain times.
Key facts
- Light year: the distance light travels in a year, about 9.46 trillion km; sunlight takes about 8 minutes 20 seconds to reach the Earth.
- Sun's layers from inside: core (about 15 million degrees C, hydrogen fused into helium), radiative zone, convective zone, photosphere (visible surface), chromosphere, corona (seen in a total solar eclipse).
- Mercury: smallest, closest to the Sun, no moon, 88-day year. Venus: hottest and brightest planet, spins east to west, no moon, Earth's twin.
- Mars: Red Planet (iron oxide), moons Phobos and Deimos, Olympus Mons; Mangalyaan entered Mars orbit on 24 September 2014 at the first attempt.
- Jupiter: largest planet, fastest spin (about 10 hours), Great Red Spot; Ganymede is the largest moon in the solar system. Saturn: least dense; its moon Titan has a thick atmosphere.
- Uranus: discovered by William Herschel in 1781, spins on its side (tilt about 98 degrees). Neptune: found in 1846 by calculation, about 165-year orbit, moon Triton.
- Dwarf planets recognised by the IAU: Ceres, Pluto, Eris, Haumea, Makemake. Pluto was reclassified in August 2006 at Prague; its largest moon is Charon.
- Halley's Comet returns about every 76 years (seen in 1986, next in 2061); a comet's tail always points away from the Sun.
- Solar eclipse: new moon, Moon between Sun and Earth; annular when the Moon looks too small. Lunar eclipse: full moon, Earth between Sun and Moon; the Moon looks red.
Quick revision
Big Bang about 13.8 billion years ago; Milky Way a barred spiral galaxy, Andromeda the nearest large galaxy; light year about 9.46 trillion km; Proxima Centauri nearest star after the Sun; Sun holds 99.8% of mass, core about 15 million C, corona seen in total eclipse, sunspot cycle 11 years; Mercury smallest, Venus hottest and spins backwards, Jupiter largest, Saturn least dense, Uranus on its side; Ganymede largest moon; Pluto a dwarf planet since 2006; Halley every 76 years; Moon's orbit tilted 5 degrees.