The time taken by the sun to revolve around the centre of our galaxy is

1. The Origin and Structure of the Universe (basic)

Welcome to your first step in mastering the cosmos! To understand the universe, we must look back roughly 13.8 billion years. The prevailing scientific model for this beginning is the Big Bang Theory. Contrary to popular belief, it wasn't an explosion in space, but rather a rapid expansion of space itself. Everything we see today was once compressed into a singularity — a point of infinite density and heat from which the universe has been stretching in all directions ever since Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.1. How do we know this happened? Scientists rely on three powerful pieces of evidence: Cosmological Redshift (observing that galaxies are moving away from us), Gravitational Waves, and most importantly, the Cosmic Microwave Background (CMB) radiation Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.3. The CMB is often called 'relic radiation'; it is the oldest light in the universe, a faint afterglow left over from the hot, early stages of the Big Bang. It fills the entire sky and serves as a 'fossil' of the universe's infancy Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.4. As the universe expanded and cooled, matter began to clump together to form galaxies, which are massive systems of stars, gas, and dust. Our home, the Milky Way, is a spiral galaxy. Within it, even the Sun is constantly in motion. It takes the Sun approximately 220 to 250 million years to complete one full revolution around the Galactic Centre — a period we call a 'Galactic Year' Physical Geography by PMF IAS, Our Galaxy (The Milky Way), p.9. Understanding this scale helps us appreciate that the universe is not static; it is a dynamic, evolving structure.

Comparing the Early vs. Modern Universe

Feature	Early Universe (Singularity/Start)	Modern Universe (Today)
Temperature	Extremely High	Near Absolute Zero (2.7 Kelvin)
Density	Infinite/Extremely High	Very Low (mostly empty space)
State	Condensed Energy/Plasma	Organized into Stars, Planets, and Galaxies

Sources: Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.1; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.3; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.4; Physical Geography by PMF IAS, Our Galaxy (The Milky Way), p.9

2. Galaxies: Classifications and the Milky Way (basic)

In our journey through the cosmos, we find that stars are not scattered randomly but are organized into massive gravitational structures called galaxies. A galaxy is essentially a vast system containing billions of stars, interstellar gas, dust, and dark matter. While there are billions of galaxies in the observable universe, they generally fall into two primary structural categories: Spiral and Elliptical.

Spiral galaxies, like our own Milky Way, are characterized by a flat, rotating disc with a central bulge and "arms" that spiral outward. These galaxies are the "star factories" of the universe because they are well-supplied with the interstellar gas necessary for new, bright stars to form. In contrast, Elliptical galaxies are shaped like elongated spheres and contain mostly older stars with very little gas or dust, meaning their era of star formation has largely ended Physical Geography by PMF IAS, Chapter 1, p.7-8.

Feature	Spiral Galaxies	Elliptical Galaxies
Shape	Flat disc with a central bulge	Round or oval (ellipsoid)
Star Population	Mix of young, bright stars and old stars	Mostly very old stars
Star Formation	Active (rich in gas and dust)	Negligible (poor in gas and dust)

Our home, the Milky Way, is a spiral galaxy containing between 100 to 400 billion stars. It is shaped like a flat disc with a diameter of approximately 150,000 to 200,000 light-years Physical Geography by PMF IAS, Chapter 1, p.8. It is important to remember that everything in the galaxy is in motion. Just as the Earth revolves around the Sun, our entire Solar System revolves around the Galactic Center. The Sun moves at a staggering speed of about 285 km per second, yet the galaxy is so vast that it takes approximately 220 to 250 million years to complete just one revolution. This duration is often referred to as a "Galactic Year" Physical Geography by PMF IAS, Chapter 1, p.9.

Sources: Physical Geography by PMF IAS, Chapter 1: The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.7-9

3. Stellar Evolution and the Life of Stars (intermediate)

Concept: Stellar Evolution and the Life of Stars

4. Measuring Space: Units and Standards (intermediate)

When we study the cosmos, our standard terrestrial units like kilometers or miles become practically useless due to the staggering distances involved. To bring order to this vastness, astronomers use specific standards of measurement. The most fundamental unit for our local neighborhood is the Astronomical Unit (AU). An AU is defined as the average distance between the Earth and the Sun, which is approximately 149.6 million kilometers (often rounded to 150 million km for ease of calculation) Physical Geography by PMF IAS, The Solar System, p.25. This unit helps us grasp the scale of our Solar System; for instance, while Earth is 1 AU from the Sun, Voyager 1 has traveled over 129 AU into the deep reaches of space Physical Geography by PMF IAS, The Solar System, p.39.

As we step outside our Solar System, we transition to the Light Year (LY). It is a common misconception that a light year is a unit of time; however, it is strictly a measure of distance. It represents the distance light travels in a vacuum in one Earth year. Given that light moves at a blistering speed of 300,000 km/second, one light year equates to roughly 9.461 × 10¹² km FUNDAMENTALS OF PHYSICAL GEOGRAPHY (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.14. To put this in perspective, light from the Sun reaches Earth in about 8.311 minutes, meaning the Sun is roughly 8.3 light-minutes away.

Finally, we must consider the movement of our entire Solar System within the Milky Way galaxy. Just as the Earth orbits the Sun, our Sun orbits the Galactic Centre. This immense journey takes a significant amount of time, known as a Galactic Year (or Cosmic Year). Current scientific estimates place one revolution of the Sun around the Milky Way at approximately 225 to 250 million years Physical Geography by PMF IAS, Our Galaxy (The Milky Way), p.9. This means the last time Earth was in this same position in the galaxy, dinosaurs were just beginning to appear!

Unit	Type	Approximate Value
Astronomical Unit (AU)	Distance	150 million km (Earth-Sun distance)
Light Year (LY)	Distance	9.46 trillion km (Distance light travels in 1 year)
Galactic Year	Time	250 million years (Sun's orbit around the Galaxy)

Sources: Physical Geography by PMF IAS, The Solar System, p.25, 39; FUNDAMENTALS OF PHYSICAL GEOGRAPHY (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.14; Physical Geography by PMF IAS, Our Galaxy (The Milky Way), p.9

5. Solar System Dynamics: Rotation and Revolution (intermediate)

To understand how our celestial neighborhood functions, we must distinguish between two fundamental movements: Rotation (spinning on an internal axis) and Revolution (traveling in a path around another body). In our Solar System, these motions are remarkably synchronized but contain fascinating anomalies. Most planets revolve around the Sun in a counter-clockwise direction (prograde) when viewed from the north pole, a direction shared by the Sun's own rotation Physical Geography by PMF IAS, The Solar System, p.25. However, while six planets also rotate counter-clockwise, Venus and Uranus exhibit retrograde rotation, meaning they spin clockwise Physical Geography by PMF IAS, The Solar System, p.25. This clockwise spin on Venus is so slow that its 'day' (rotation) actually lasts longer than its 'year' (revolution).

The mechanics of these orbits are governed by Kepler’s Laws of Planetary Motion. Contrary to popular belief, orbits are not perfect circles but ellipses with the Sun at one focus Physical Geography by PMF IAS, The Solar System, p.21. A critical consequence of this is that a planet's orbital velocity is not constant. According to Kepler’s Second Law, a planet moves faster when it is closer to the Sun and slower when it is further away. For Earth, this means we move slowest during the Northern Hemisphere summer when we are furthest from the Sun, making our summers approximately three days longer than our winters Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.256.

Finally, we must zoom out to see the 'big picture.' The Sun itself is not a fixed point in the universe; it is in constant motion, revolving around the center of the Milky Way galaxy. This massive journey, known as a Galactic Year, takes approximately 250 million years to complete Physical Geography by PMF IAS, The Universe, p.9. This means that since the time of the first dinosaurs, the Solar System has completed only one full lap around the galaxy.

Feature	Rotation	Revolution
Definition	Spinning on its own axis	Movement around the Sun
Primary Effect	Day and Night cycle	Seasons and Year length
Earth's Duration	~24 hours	365.25 days
Standard Direction	Counter-clockwise (West to East)	Counter-clockwise

Sources: Physical Geography by PMF IAS, The Solar System, p.21, 25; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.256; Physical Geography by PMF IAS, The Universe, p.9

6. The Sun’s Motion within the Galaxy (exam-level)

While we often think of the Sun as a fixed center of our solar system, it is actually in constant motion. Just as the Earth completes one revolution around the Sun in approximately 365 days Science-Class VII, Earth, Moon, and the Sun, p.176, the Sun itself revolves around the Galactic Center of the Milky Way. This orbital path is not a perfect circle but a complex trajectory influenced by the combined mass of the stars, gas, and dark matter within our galaxy.

The Sun is located about 26,000 to 28,000 light-years away from the Galactic Center, positioned in the Orion Arm of the Milky Way. To maintain its orbit at this distance, the Sun travels at a blistering speed of approximately 220 to 230 kilometers per second. Despite this incredible velocity, the Milky Way is so vast that it takes an immense amount of time to complete just one single lap.

This period of one complete revolution is known as a Galactic Year (or Cosmic Year). Current astronomical estimates place this duration at roughly 225 to 250 million years Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.9. To put this in perspective, the last time the Sun was in its current position relative to the galaxy, the first dinosaurs were just beginning to appear on Earth!

Feature	Earth's Orbit	Sun's Orbit (Galactic Year)
Center of Orbit	The Sun	The Galactic Center (Milky Way)
Orbital Speed	~30 km/s	~220-230 km/s
Time Taken	365.25 Days	~250 Million Years

Sources: Science-Class VII, Earth, Moon, and the Sun, p.176; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.9

7. The Galactic Year (Cosmic Year) (exam-level)

While we are accustomed to the Terrestrial Year — the 365.25 days it takes for Earth to complete one revolution around the Sun Certificate Physical and Human Geography, The Earth's Crust, p.6 — the Sun itself is also in constant motion. Our Solar System is located in the Orion Arm of the Milky Way, a vast spiral galaxy. Just as planets orbit stars, our Sun orbits the Galactic Centre (the barycenter of the Milky Way). This immense orbital period is known as the Galactic Year or Cosmic Year.

The scale of this movement is staggering. To complete this journey, the Sun travels at an average velocity of approximately 285 km per second Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.9. Despite this incredible speed, the Milky Way is so massive (with a diameter of 150,000 to 200,000 light-years) that it takes the Sun roughly 220 to 250 million years to finish just one lap Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.8-9.

To put this in perspective, the last time the Sun was in its current position in the galaxy, the first dinosaurs were just beginning to appear on Earth. Since its formation about 4.6 billion years ago, the Sun has completed only about 20 of these "Galactic Years." Understanding this concept helps astronomers map the mass distribution of our galaxy and study the gravitational forces exerted by the supermassive black hole at the center of the Milky Way.

Feature	Earth's Revolution	Sun's (Solar System's) Revolution
Center of Orbit	The Sun	The Galactic Centre
Orbital Speed	~30 km/second	~285 km/second
Time Period	1 Year (365.25 days)	1 Galactic Year (~250 million years)

Sources: Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.8-9; Certificate Physical and Human Geography, The Earth's Crust, p.6

8. Solving the Original PYQ (exam-level)

Now that you have mastered the structure of the Milky Way and the lifecycle of stars, this question tests your ability to apply galactic dynamics to a specific timeframe. Just as the Earth orbits the Sun, our Sun itself is in constant motion, revolving around the Galactic Centre. This specific orbital period is known as the Galactic Year or Cosmic Year. To answer this correctly, you must bridge the gap between knowing that the Sun is moving and knowing the scale of that movement relative to the size of our spiral galaxy.

To arrive at the correct answer, look for the standard textbook estimate for this revolution. While various astronomical measurements range between 220 and 230 million years, general studies sources like Physical Geography by PMF IAS and Astronomy 2e (OpenStax) typically use 250 million years as a rounded, representative figure. In the context of UPSC, when precise scientific values vary slightly, the most frequently cited rounded estimate in standard literature is your safest bet. Therefore, (C) 250 million years is the logical choice.

UPSC often includes options that test your sense of astronomical scale. Options (A) 25 million and (B) 100 million years are common traps that underestimate the vast distance—roughly 27,000 light-years—the Sun must travel to complete one circuit. Conversely, (D) 500 million years is an exaggeration of that duration. By recognizing that the Sun travels at a velocity of about 230 km/s, you can eliminate these extremes and identify that a value in the mid-200 millions is the only one that aligns with the known physics of our galaxy.