3.1 Universe, solar system and the Earth notes

1. 3.1.1Origin of the universe: Big Bang and the expanding universe

   The universe began about 13.8 billion years ago in a hot, dense state that has been expanding ever since (the Big Bang); our solar system and the Earth formed much later, about 4.6 billion years ago, from a rotating cloud of gas and dust.

   Standard · 9 min · Free in the app
2. 3.1.2Stars and galaxies: life cycle of a star and the Milky Way

   Stars are huge balls of hot gas that shine by nuclear fusion; their mass decides whether they end as white dwarfs, neutron stars or black holes, and billions of them together form galaxies such as our Milky Way.

   Standard · 8 min · Free in the app
3. 3.1.3The Sun and the eight planets: key features compared

   The Sun holds almost all the mass of the solar system and drives it with its energy; around it move eight planets, four small rocky inner planets and four giant outer planets, each with a few features that exams ask again and again.

   Standard · 8 min · Free in the app
4. 3.1.4Dwarf planets, asteroids, comets and meteors

   Besides the eight planets, the solar system has dwarf planets such as Pluto, rocky asteroids mostly between Mars and Jupiter, icy comets from its outer edge, and small fragments that become meteors and meteorites when they meet the Earth.

   Standard · 8 min · Free in the app
5. 3.1.5The Moon: phases, tides link and solar and lunar eclipses

   The Moon, the Earth's only natural satellite, always shows us the same face, goes through phases as its lit half turns towards or away from us, raises the main tides, and causes eclipses when the Sun, Earth and Moon line up.

   Standard · 9 min · Free in the app
6. 3.1.6Earth's rotation and revolution: day and night, seasons, solstices and equinoxes

   The Earth spins on a tilted axis once a day, giving day and night, and goes round the Sun once a year; because the tilted axis keeps pointing the same way, the Sun's overhead position shifts between the tropics, giving seasons, solstices and equinoxes.

   Standard · 8 min · Free in the app
7. 3.1.7Latitudes and longitudes: important parallels and heat zones

   Latitudes (parallels) measure distance north or south of the equator and mark the tropics and polar circles that define the heat zones; longitudes (meridians) measure distance east or west of the Prime Meridian at Greenwich and form, with parallels, the grid used to fix any place on Earth.

   Standard · 8 min · Free in the app
8. 3.1.8Longitude and time: local time, standard time, IST and the International Date Line

   Because the Earth turns 15° every hour, local time changes by four minutes for every degree of longitude; countries therefore adopt a standard meridian for a common clock (India uses 82°30′ E, UTC +5:30), and the date changes at the International Date Line near 180°.

   Standard · 8 min · Free in the app

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