Gravity and Space: Why Things Fall and Planets Orbit

According to a famous story, Isaac Newton was prompted to think about gravity when he saw an apple fall from a tree. The story is probably embellished, but the question he asked was real and enormous: if gravity pulls an apple to the ground, could the same force also hold the Moon in its orbit? His answer, published in 1687, was yes, and it unified the physics of falling objects on Earth with the motion of the Moon, the planets and the tides. Gravity is the force that shapes the universe: it pulled the Sun together from a cloud of gas, keeps the planets in orbit, and holds galaxies together. This article explains how gravity works, how orbits and satellites work, and what we know about the Solar System, the stars and the universe.

1.The Force of Gravity

Gravity is the force of attraction between any two objects that have mass. It is always attractive, it acts over any distance (becoming weaker as the distance grows), and it is by far the weakest of the fundamental forces. We notice it only when one of the masses is enormous, such as a planet. Newton's law of universal gravitation gives the force between two masses:

2.Gravitational Field Strength and Weight

Around any mass there is a gravitational field. The gravitational field strength g at a point is the force on each kilogram placed there (N/kg), which is also the free-fall acceleration (m/s²). For a planet of mass M and radius R, at its surface:

Even on Earth, g is not exactly the same everywhere: it is slightly smaller at the equator (about 9.78) than at the poles (about 9.83), because of the Earth's rotation and slightly flattened shape, and it decreases with altitude. The standard value used for reference is 9.80665 N/kg.

3.Orbits and Satellites

Newton imagined a cannon on a very high mountain. A ball fired slowly lands nearby. A ball fired faster lands farther away. Since the Earth's surface curves away, there is a speed at which the ball falls towards the Earth but keeps missing it; it is then in orbit. An orbit is a continuous free fall around a body. The only force is gravity, and it supplies the centripetal force that bends the path into a circle or ellipse.

For a circular orbit of radius r around a mass M, setting gravity equal to the centripetal requirement gives:

• Low Earth orbit (a few hundred to about 2000 km): the ISS, Earth-observation and many communication satellites.
• Navigation satellites: GPS satellites orbit at about 20 200 km with a period of about 12 hours. Receiving signals from several of them lets a receiver compute its position.
• Geostationary orbit: at about 35 786 km above the equator the period is 24 hours, matching the Earth's rotation, so the satellite stays above one point of the Earth. Used for TV broadcasting and weather satellites.

4.Kepler's Laws

Before Newton, the German astronomer Johannes Kepler (1571–1630) analysed careful observations of the planets and found three patterns, published between 1609 and 1619:

1. Planets move in ellipses with the Sun at one focus (not in perfect circles).
2. A line from the Sun to a planet sweeps out equal areas in equal times, so a planet moves faster when it is nearer the Sun.
3. The square of a planet's orbital period is proportional to the cube of its average distance from the Sun: T² ∝ r³.

5.The Solar System

The Solar System is the Sun and everything bound to it by gravity. The Sun contains about 99.86% of the total mass. It is a star: a huge ball of hot gas, mainly hydrogen and helium, in which nuclear fusion releases energy. About 4.6 billion years ago it formed, with the planets, from a collapsing cloud of gas and dust.

One astronomical unit (AU) is the average Earth–Sun distance: 149 597 870 700 m, about 150 million km. Light from the Sun takes about 8.3 minutes to reach us.

• Rocky (terrestrial) planets are small, dense and close to the Sun. Venus has a thick carbon dioxide atmosphere and a surface hot enough to melt lead, so it is the hottest planet, hotter even than Mercury.
• Gas giants (Jupiter, Saturn) are enormous and mostly hydrogen and helium. Ice giants (Uranus, Neptune) contain more "ices" such as water, ammonia and methane around a rocky core.
• Dwarf planets such as Pluto, Ceres and Eris are round bodies that have not cleared their orbits of other objects. In 2006 the International Astronomical Union (IAU) adopted the definition that placed Pluto in this class.
• Asteroids are rocky bodies, mostly in the belt between Mars and Jupiter. Comets are balls of ice and dust whose tails of gas and dust form when they approach the Sun. Beyond Neptune lie the Kuiper belt and, further out, the hypothesised Oort cloud.

6.The Moon: Phases, Eclipses and Tides

The Moon orbits the Earth once every 27.3 days, and takes about 29.5 days to go through its cycle of phases (the phases take longer because the Earth also moves round the Sun). Half of the Moon is always lit by the Sun. What changes is how much of the lit half we can see from Earth. The Moon always shows nearly the same face to us because it rotates once for each orbit.

• Solar eclipse: the Moon passes between the Sun and Earth (at new Moon) and its shadow falls on part of the Earth. Total eclipses are seen only in a narrow path where the Moon appears just large enough to cover the Sun. Never look at the Sun directly; use certified eclipse glasses.
• Lunar eclipse: the Earth passes between the Sun and the Moon (at full Moon) and its shadow falls on the Moon, often turning it a coppery red. It is safe to watch with the naked eye.
• Eclipses do not happen every month because the Moon's orbit is tilted by about 5° to the Earth's orbit, so the three bodies seldom line up exactly.
• Tides: the Moon's gravity pulls more strongly on the near side of the Earth than on its centre and far side, producing two tidal bulges in the oceans, so most coasts have two high tides a day. The Sun adds a smaller effect. At new and full Moon the Sun and Moon pull in line, giving larger spring tides; at the quarter Moons they pull at right angles, giving smaller neap tides.

7.Day, Night and the Seasons

The Earth spins on its axis once in about 24 hours (precisely, 23 hours 56 minutes relative to the stars), which gives day and night. It orbits the Sun once in about 365.25 days. The Earth's axis is tilted by about 23.4° to the plane of its orbit, and it points in nearly the same direction in space all year. This tilt, not the distance from the Sun, causes the seasons. (In fact the Earth is nearest to the Sun in early January, during the northern winter.)

When a hemisphere is tilted towards the Sun, the Sun climbs higher in the sky, sunlight strikes the ground more directly (so the same energy is spread over a smaller area), and the days are longer. Near the poles there are weeks of continuous daylight in summer and continuous darkness in winter.

8.Stars and Their Life Cycles

Stars form inside giant clouds of gas and dust called nebulae. Gravity pulls a clump together; as it shrinks it heats up until the core is hot and dense enough for hydrogen to fuse into helium. At that point the star begins its long main sequence life, in which the outward pressure of the hot gas balances the inward pull of gravity. Our Sun is about halfway through this stage, with roughly five billion years to go.

What happens next depends on the star's mass:

• Stars like the Sun: when the hydrogen in the core is used up, the star swells into a red giant, then sheds its outer layers as a planetary nebula, leaving a hot, Earth-sized core called a white dwarf that slowly cools.
• Much more massive stars: they burn fuel quickly, expand into red supergiants, and end in a violent explosion called a supernova. The core that remains collapses into a neutron star or, if massive enough, a black hole, an object whose gravity is so strong that nothing, not even light, can escape from inside its event horizon.

Stars are very far away. The nearest star after the Sun, Proxima Centauri, is about 4.2 light-years away: its light takes about 4.2 years to reach us, even though light covers 300 000 km every second. When we look at stars, we see them as they were in the past.

9.Galaxies and the Universe

Stars are grouped into huge systems called galaxies, held together by gravity. Our Sun is one of hundreds of billions of stars in the Milky Way, a spiral galaxy about 100 000 light-years across. The Sun orbits the Milky Way's centre once every 220–250 million years. There are billions of other galaxies; the nearest large one, Andromeda, is about 2.5 million light-years away.

In the 1920s Edwin Hubble showed that distant galaxies are moving away from us, and the farther away a galaxy is, the faster it recedes. The light from distant galaxies is stretched to longer wavelengths, an effect called redshift. This tells us that the universe is expanding. Running the expansion backwards leads to the Big Bang theory: the universe began in an extremely hot, dense state about 13.8 billion years ago. A major piece of evidence is the cosmic microwave background, faint microwave radiation filling the whole sky, discovered in 1965 by Arno Penzias and Robert Wilson. Astronomers also find that visible matter is only a small part of the universe: most of it appears to be dark matter and dark energy, whose nature is still unknown.

10.Exploring Space

Space missions are used for telecommunications, weather forecasting, navigation, mapping resources and monitoring climate, as well as for science. Rockets work by Newton's third law, as described in the article on force and motion. NASA, ESA, SUPARCO and other agencies publish current mission schedules, including plans to return people to the Moon, so check their websites for the latest news.

11.Common Mistakes to Avoid

12.Check Your Understanding

Try each question before opening the answer.

Summary

• Gravity is an attractive force between all masses: F = Gm₁m₂ ÷ r², an inverse-square law. Field strength g = GM ÷ r².
• Orbits are continuous free fall; v = √(GM ÷ r). Kepler: elliptical orbits, equal areas in equal times, T² ∝ r³.
• The Solar System has eight planets, dwarf planets, asteroids and comets, all orbiting the Sun.
• The Moon's phases, eclipses, tides, day and night, and the seasons all follow from the geometry of orbits and the Earth's tilt.
• Stars form in nebulae and end as white dwarfs, neutron stars or black holes. The universe is expanding and about 13.8 billion years old.

Keep Learning on EDUSAMBAM

Sources and Further Reading

This article is original EDUSAMBAM writing. Definitions, constants and facts were checked against the sources below. Diagrams are original and were not copied from any source.

• NASA Solar System Exploration: overview of the planets, Moon, Sun and missions. NASA — Our Solar System.
• NASA / NSSDC: planetary and lunar fact sheets (masses, radii, surface gravity, orbital periods). NASA — Planetary Fact Sheet.
• NIST: the Newtonian constant of gravitation G and the standard acceleration of gravity. NIST — Fundamental Physical Constants.
• International Astronomical Union: the 2006 definition of a planet and the dwarf planet class. IAU.
• OpenStax, University Physics Volume 1: gravitation. OpenStax — University Physics Volume 1. OpenStax, Astronomy 2e: OpenStax — Astronomy 2e.
• ESA and SUPARCO: space agency news and mission information. ESA; SUPARCO.
• Encyclopaedia Britannica: Johannes Kepler; Isaac Newton.
• Further reading kept from the original article: Gravity — Encyclopaedia Britannica.
• Further reading kept from the original article: Orbits and Kepler's Laws — NASA Science.
• Further reading kept from the original article: Physics — CERN.

Note on the numbers: planetary data are rounded; cosmological values such as the age of the universe are current best estimates and may be refined. Please check numerical values against your own syllabus or exam board specification before relying on them.