Articles

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Cell and Molecular Biology

The Atom: Nucleus, Electrons, Isotopes

The nucleus holds almost all of an atom's mass; the electrons hold almost all of its volume. This article works through the three numbers behind that structure — the proton count Z that fixes the element, the mass number A that separates the isotopes, and the mass missing from ¹²C — using CODATA 2022, NIST and IAEA data.

3 min read6

Mechanics

Mass, Inertia and Weight

Mass and inertia are two ways of describing one property; weight is a force, measured in newtons, that changes with where you stand. This article sets the three quantities side by side as NIST and BIPM define them, explains why "weightless" does not mean gravity is gone, and why the equality of inertial and gravitational mass remains an open question.

2 min read5

Mechanics

Newton's Three Laws of Motion

Kepler said how the planets move; Newton said why. This article gives the three laws as NASA renders them from the Principia, the rigorous form of the second law (force as the rate of change of momentum), the point where the third law is most often misread, and the range over which the three laws still hold.

2 min read5

Heat and Energy

What Is Energy?

Energy is a scalar quantity measured in joules, and in a closed system that number does not change. This article covers the working definition and where it falls short, the SI unit, the main forms of energy, and why the total stays the same through every conversion.

2 min read5

Observing the Sky

The cosmic distance ladder: how we measure stars and galaxies

No single method stands alone: parallax calibrates Cepheids, Cepheids calibrate Type Ia supernovae, and those calibrate the Hubble constant — which is why a disagreement at the top forces a re-check of every rung below.

3 min read9

The Solar System

Comets: origin, structure and fate

Barely heated or altered in 4.6 billion years, comets are the closest thing we have to the raw material the planets were built from — and Rosetta's in-situ data revised a good deal of what was believed about them.

3 min read2

Modern Physics

Gravitational waves and gravity waves: one word apart, nothing in common

A gravitational wave is a ripple in spacetime that needs no medium; a gravity wave is a mechanical oscillation of water or air that cannot exist without one. The two are routinely confused in translation.

2 min read3

Observing the Sky

Solar and lunar eclipses: two shadows, not one phenomenon

A solar eclipse is the Moon's shadow sweeping across Earth; a lunar eclipse is the Moon entering Earth's shadow. That difference sets how wide the viewing zone is, how long it lasts, and whether eye protection is required.

3 min read1

Observing the Sky

Aurora: light made by Earth's own atmosphere

What arrives from the Sun is energy; the light itself comes from oxygen atoms and nitrogen molecules high in the atmosphere after electrons strike them — and the lifetime of each excited state is why each altitude has its own colour.

2 min read2

Atmosphere and Weather

What causes the seasons: axial tilt, not distance

Earth is closest to the Sun in early January — the middle of the northern winter. Seasons come not from how much energy arrives but from how the 23.44° axial tilt distributes it between hemispheres and across the year.

3 min read1

The Solar System

How the Solar System was born

It all began with a collapsing cloud of gas and dust 4.57 billion years ago. The hardest question is not how planets formed, but why the Sun holds 99.86% of the mass yet only 1% of the angular momentum.

6 min read2

Cosmology

Dark matter and dark energy: two different things, both invisible

They share only the word "dark" and the fact that nobody has seen either. One pulls matter together, the other pushes space apart. Together they are 95% of the universe, and we do not know what either one is.

3 min read3