The Universe and the Earth — Quick Revision

A last-week revision sheet for the Earth in space. Most of these lines appear in Prelims as statements to be judged true or false.

The Universe in Five Lines

Concept	Key point
Big Bang theory	The universe expanded from an extremely hot, dense state about 13.8 billion years ago
Evidence of expansion	Edwin Hubble (1929): galaxies are moving away from us, faster the farther they are
Light year	A unit of distance — how far light travels in a year (about 9.46 trillion km)
Our galaxy	The Milky Way, a spiral galaxy; the Sun lies in one of its arms, far from the centre
Age of the Earth	About 4.6 billion years

Planet	Remember it for
Mercury	Closest to the Sun; smallest planet; no moon
Venus	Hottest planet (thick CO₂ atmosphere, runaway greenhouse); rotates east to west; its day is longer than its year; no moon
Earth	Densest planet; only known planet with life
Mars	Red due to iron oxide; Olympus Mons, the largest known volcano in the solar system
Jupiter	Largest planet; Great Red Spot (a giant storm)
Saturn	Prominent ring system; density lower than water
Uranus	Axis tilted about 98° — it “rolls” on its side
Neptune	Farthest planet from the Sun

Inner (terrestrial) planets: Mercury, Venus, Earth, Mars — rocky and dense.
Outer (Jovian) planets: Jupiter, Saturn, Uranus, Neptune — large, gaseous or icy, low density.
Asteroid belt: between Mars and Jupiter.
Pluto was reclassified as a dwarf planet by the International Astronomical Union in August 2006. A planet must orbit the Sun, be nearly round, and have cleared its orbital neighbourhood — Pluto fails the third test.
Sunlight takes about 8 minutes 20 seconds to reach the Earth; the mean Earth–Sun distance (1 astronomical unit) is about 150 million km.

High-yield facts

Venus and Uranus show retrograde (east-to-west) rotation; most planets rotate west to east.
The Moon always shows the same face because its rotation period equals its revolution period (about 27.3 days) — synchronous rotation.
A solar eclipse can occur only at new moon; a lunar eclipse only at full moon.
The Earth is an oblate spheroid (geoid): its polar diameter is shorter than its equatorial diameter.

Motion	Period	Direction	Consequence
Rotation	About 24 hours	West to east	Day and night; Coriolis effect; the Sun appears to rise in the east
Revolution	About 365¼ days	Anticlockwise when viewed from above the North Pole	Seasons (with tilt); the leap year every fourth year

Axis tilted about 23.5° from the perpendicular (about 66.5° with the orbital plane), always pointing the same way — parallelism of the axis.
Perihelion (closest, about 147 million km) ≈ 3 January; aphelion (farthest, about 152 million km) ≈ 4 July.
Circle of illumination — the line dividing day from night.

Item	Value
1° of latitude	≈ 111 km everywhere
1° of longitude	≈ 111 km at the Equator, zero at the poles
Time	15° = 1 hour; 1° = 4 minutes; east is ahead
Prime Meridian	Greenwich; adopted at the 1884 International Meridian Conference (Washington)
IST meridian	82°30′ E (near Mirzapur, UP); UTC + 5:30
India’s extent (mainland)	~8°4′ N–37°6′ N; ~68°7′ E–97°25′ E
Tropic of Cancer in India	8 states: Gujarat, Rajasthan, MP, Chhattisgarh, Jharkhand, West Bengal, Tripura, Mizoram
International Date Line	~180°; westward crossing +1 day, eastward −1 day

Traps to avoid

Light year is a distance, not a time.
Distance from the Sun does not cause the seasons; tilt plus revolution does.
Rotation causes day and night; revolution (with tilt) causes seasons.
Only the Equator among parallels is a great circle; every meridian is half of one.
Crossing the IDL westward advances the date — the opposite of what intuition suggests.