The universe is a vast expanse of space containing all matter and energy in existence. Scientists believe it is expanding outward, and different models have been proposed to explain its structure.

The Big Bang Theory, proposed by George Lemaitre, explains the origin of the universe around 13.8 billion years ago from a singularity (infinite mass, zero volume). It rapidly expanded, forming the present universe. Supporting evidences include red shift of galaxies and cosmic microwave background radiation.

Galaxies are vast systems of gas, dust, and billions of stars held together by gravity. They exist in different shapes – spiral, elliptical, and irregular. Our solar system is located in the Milky Way, a spiral galaxy containing a supermassive black hole (Sagittarius A*) at its center.

Stars are giant glowing balls of gas (mostly hydrogen and helium) held by gravity. Their color depends on temperature – blue stars are hottest, red are coolest. Constellations are recognizable patterns of stars, with 88 officially recognized ones, used for naming, navigation, and astronomical references.

The Solar System is a gravitationally bound system of the Sun and the celestial objects that orbit it. The space beyond the solar system is known as interstellar space. Multiple theories explain its origin and evolution, ranging from Kant’s Gaseous Hypothesis to Hoyle’s Supernova Hypothesis.

The Sun is the central star of the Solar System, around 5 billion years old. Composed mainly of hydrogen and helium, it has a diameter of 1.392 million km. Its temperature ranges from 15 million °C at the core to 5,500 °C at the surface. The Sun has distinct inner and outer layers, each responsible for energy generation and radiation.

Several dynamic phenomena are associated with the Sun such as sunspots, solar wind, coronal mass ejections (CME), and solar cycles. These impact Earth’s magnetosphere and atmosphere, producing effects like auroras and STEVE.

Planets are celestial objects that orbit stars in elliptical paths. Apart from classical planets, there are dwarf planets, exoplanets, and protoplanets. Pluto, once considered the ninth planet, is now categorized as the largest dwarf planet with unique features.

Planets are classified into Inner (Terrestrial) and Outer (Jovian) planets. Inner planets (Mercury, Venus, Earth, Mars) are rocky, dense, with solid surfaces and iron cores. Outer planets (Jupiter, Saturn, Uranus, Neptune) are gas giants, less dense, and lack solid surfaces.

Each planet in the Solar System has unique features related to rotation, revolution, moons, rings, and physical characteristics. While Mercury is the fastest planet, Venus is the hottest, Earth supports life, Mars appears red, and Jupiter is the largest. Saturn has magnificent rings, Uranus is called an Ice Giant, and Neptune is Uranus’s twin.

Beyond Neptune lies the Kuiper Belt, a region of icy remnants from Solar System formation. Further out is the Oort Cloud, a spherical shell of comets. Asteroids are rocky, smaller bodies with elliptical orbits, while comets are icy objects with eccentric orbits that develop characteristic tails when near the Sun.

Meteoroids are small space rocks. When they enter Earth’s atmosphere, they become meteors (shooting stars). If they survive and reach Earth’s surface, they are called meteorites. Meteor showers occur when Earth passes through a stream of meteoroids. These phenomena provide insights into the Solar System’s composition and evolution.

The Geological Time Scale (GTS) is the calendar of Earth's history, dividing it into eons, eras, periods, epochs, and ages. It helps trace the evolution of life, flora, and fauna across billions of years.

The geographical grid is a network of imaginary lines—latitude and longitude—used to determine exact locations on Earth. Latitudes measure north-south distance from the Equator, while longitudes measure east-west distance from the Prime Meridian.

Latitudes and longitudes form the geographical grid, the basis of navigation and location mapping. Important reference lines include the Equator, Tropic of Cancer, Tropic of Capricorn, Arctic and Antarctic Circles, Prime Meridian, and International Date Line. These define hemispheres, time zones, and climatic regions of the Earth.

The Earth has four key motions: rotation, revolution, axial tilt, and polar motion. Rotation defines day-night and Coriolis Effect, revolution defines year and seasons, tilt defines solstices/equinoxes, and polar motion reflects Earth’s dynamic axis shifts. Human activities (glacier melting, groundwater extraction) are now altering Earth’s axis.

Earth experiences four major seasons—Summer, Winter, Spring, and Autumn—caused by its revolution around the Sun and axial tilt (23.5°). While one hemisphere has summer, the other has winter. Solstices and equinoxes mark the transition points. Polar regions experience continuous day or night for six months. Seasonal cycles are also linked with agriculture, monsoon, and cultural traditions.