Questions on the Sun, planets, the Moon, eclipses and space missions appear regularly in the general science part of Lok Sewa papers, and Nepal’s own satellite is a favourite current-affairs question. This note explains the solar system step by step, compares the eight planets in a table, explains phases and eclipses, and lists the space missions and firsts that examiners ask about most.
1. The universe, galaxy and solar system
The universe is everything that exists: all space, matter and energy. Stars are grouped in huge systems called galaxies. Our galaxy is the Milky Way, a spiral galaxy. The nearest large galaxy to it is Andromeda.
The solar system is made of the Sun and everything that moves around it: eight planets, their moons, dwarf planets, asteroids, comets and dust. It formed about 4.6 billion years ago.
| Unit | Meaning |
|---|---|
| Astronomical Unit (AU) | Average distance between the Earth and the Sun, about 150 million km |
| Light-year | Distance light travels in one year, about 9.46 trillion km (a unit of distance, not time) |
| Speed of light | About 3 lakh km per second (300,000 km/s) |
2. The Sun
- The Sun is a star, a huge ball of hot gas. It is the nearest star to the Earth.
- It holds about 99.8 percent of the total mass of the solar system.
- It is made mostly of hydrogen, followed by helium.
- It produces energy by nuclear fusion: hydrogen nuclei join to form helium and release huge energy.
- Its visible surface (photosphere) is about 5,500 °C, while the core is about 15 million °C.
- Sunlight takes about 8 minutes 20 seconds to reach the Earth.
- Sunspots are darker, cooler patches on the surface. The outer layer, the corona, is seen during a total solar eclipse.
3. The eight planets
In order from the Sun the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune (memory line: My Very Educated Mother Just Served Us Noodles). The first four are small, rocky inner (terrestrial) planets. The last four are large outer planets: Jupiter and Saturn are gas giants, and Uranus and Neptune are ice giants.
| Planet | Time to go round the Sun | Key fact asked in exams |
|---|---|---|
| Mercury | 88 days | Smallest planet and closest to the Sun; fastest orbit; no moon |
| Venus | 225 days | Hottest planet (thick carbon dioxide atmosphere); brightest planet, called the morning and evening star; spins in the opposite direction; no moon |
| Earth | 365.25 days | Only planet known to have life; called the blue planet |
| Mars | 687 days | Red planet (iron oxide in the soil); two small moons, Phobos and Deimos; Olympus Mons, the largest volcano in the solar system |
| Jupiter | About 12 years | Largest planet; Great Red Spot (a giant storm); Ganymede, its moon, is the largest moon in the solar system |
| Saturn | About 29.5 years | Famous bright rings; least dense planet; Titan is its largest moon |
| Uranus | About 84 years | Rotates on its side; discovered by William Herschel in 1781 |
| Neptune | About 165 years | Farthest planet from the Sun; strongest winds; discovered in 1846 |
In 2006 the International Astronomical Union (IAU) reclassified Pluto as a dwarf planet, so the solar system now has eight planets.
4. The Earth and the Moon
- Rotation of the Earth on its axis takes about 24 hours and causes day and night.
- Revolution of the Earth round the Sun takes about 365.25 days. The extra quarter day is added as 29 February every fourth year (leap year).
- The Earth’s axis is tilted by about 23.5 degrees. This tilt, together with revolution, causes the seasons.
- The Moon is the Earth’s only natural satellite, about 384,400 km away. Its diameter is about one quarter of the Earth’s and its gravity is about one sixth of the Earth’s.
- The Moon has no air and no light of its own; moonlight is reflected sunlight.
- The Moon takes about 27.3 days to go round the Earth and the same time to spin once, so we always see the same face.
- The cycle of phases (new moon to new moon) takes about 29.5 days. Waxing means growing; waning means shrinking.
- Tides in the sea are caused mainly by the Moon’s gravity.
5. Eclipses
An eclipse happens when one body comes into the shadow of another. The Sun, Earth and Moon must be in a nearly straight line.
| Point | Solar eclipse | Lunar eclipse |
|---|---|---|
| Position | Moon comes between the Sun and the Earth | Earth comes between the Sun and the Moon |
| Shadow | Moon’s shadow falls on the Earth | Earth’s shadow falls on the Moon |
| Day of the month | Only on a new moon day (Aunsi) | Only on a full moon day (Purnima) |
| Types | Total, partial and annular (a ring of sunlight) | Total, partial and penumbral |
| Safety | Never look directly at the Sun; use proper eclipse glasses | Safe to watch with the naked eye |
An annular solar eclipse happens when the Moon is farther from the Earth and looks too small to cover the Sun fully, so a bright ring is seen. During a total lunar eclipse the Moon often looks reddish, which is popularly called a blood moon. Eclipses do not happen every month because the Moon’s orbit is tilted by about 5 degrees to the Earth’s orbit. The dark central part of a shadow is the umbra and the lighter outer part is the penumbra.
6. Other members of the solar system
- Dwarf planets: Pluto, Ceres and Eris are examples. Ceres lies in the asteroid belt.
- Asteroids: rocky bodies, mostly in the asteroid belt between Mars and Jupiter.
- Comets: bodies of ice, dust and rock. Near the Sun they form a glowing tail that always points away from the Sun. Halley’s Comet returns about every 76 years; it was last seen in 1986 and is expected again in 2061.
- Meteor: a piece of rock that burns up in the Earth’s atmosphere, seen as a shooting star. A piece that reaches the ground is a meteorite.
- Constellations are groups of stars that form patterns, such as Orion and the Great Bear. Polaris (Dhruva Tara), the Pole Star, shows the north direction.
7. Famous space missions and firsts
| Year | Event |
|---|---|
| 4 October 1957 | Sputnik 1 (USSR): the first artificial satellite |
| November 1957 | Laika, a dog, became the first animal to orbit the Earth (Sputnik 2) |
| 12 April 1961 | Yuri Gagarin (USSR): the first human in space (Vostok 1) |
| 16 June 1963 | Valentina Tereshkova (USSR): the first woman in space |
| 20 July 1969 | Apollo 11 (USA): Neil Armstrong became the first person to walk on the Moon, followed by Buzz Aldrin |
| 1990 | Hubble Space Telescope placed in orbit |
| 2000 | First crew began living on the International Space Station (ISS) |
| 24 September 2014 | India’s Mangalyaan (Mars Orbiter Mission) entered Mars orbit on its first attempt |
| 25 December 2021 | James Webb Space Telescope launched |
| 23 August 2023 | India’s Chandrayaan-3 landed near the Moon’s south pole, the first landing in that region |
Major space agencies: NASA (USA), Roscosmos (Russia), ISRO (India), CNSA (China), JAXA (Japan) and ESA (Europe).
8. Satellites and Nepal in space
A satellite is a body that moves round a planet. The Moon is a natural satellite; machines placed in orbit are artificial satellites. A rocket must reach about 11.2 km/s, the Earth’s escape velocity, to leave the Earth’s gravity completely.
- Geostationary satellites orbit about 35,786 km above the equator and take 24 hours for one round, so they seem fixed in the sky. They are used for TV broadcasting, communication and weather watching.
- Low Earth orbit satellites fly a few hundred kilometres up and are used for taking pictures of the Earth and for the ISS.
- GPS and other navigation systems use groups of satellites to find exact locations.
NepaliSat-1 is Nepal’s first satellite. It was designed and built by Nepali engineers Aabhas Maskey and Hariram Shrestha at the Kyushu Institute of Technology, Japan, under the BIRDS-3 project, with funding from the Nepal Academy of Science and Technology (NAST). It was launched on 17 April 2019 (18 April, Nepal time) on an Antares rocket from Virginia, USA, to the ISS, and was later released into orbit about 400 km above the Earth. It was a small 1U CubeSat of about 1.3 kg, used to take pictures of Nepal and test communication. It re-entered the atmosphere in October 2021.
📌 Key points to remember
- The solar system formed about 4.6 billion years ago; the Sun holds about 99.8 percent of its mass.
- The Sun makes energy by nuclear fusion of hydrogen into helium; sunlight takes about 8 minutes 20 seconds to reach us.
- Mercury is the smallest and closest planet, Venus the hottest and brightest, Jupiter the largest, Neptune the farthest.
- Pluto became a dwarf planet in 2006; the asteroid belt lies between Mars and Jupiter.
- The Moon is about 384,400 km away; its phases repeat every 29.5 days and its gravity is one sixth of the Earth’s.
- Solar eclipse: Moon in the middle, on a new moon day. Lunar eclipse: Earth in the middle, on a full moon day.
- Sputnik 1 (1957), Yuri Gagarin (1961), Valentina Tereshkova (1963), Apollo 11 and Neil Armstrong (1969).
- NepaliSat-1, Nepal’s first satellite, was launched on 17 April 2019 under the BIRDS-3 project, funded by NAST.
Test yourself: Solar System and Space
50 questions on The universe, galaxy and solar system, Eclipses and Satellites and Nepal in space. Each attempt picks 25 at random.