Tag Archives: words

Dawn, dusk, sunrise, sunset and twilight

Dawn occurs before sunrise, before the top of the Sun reaches the horizon. Astronomical Dawn is the point at which it becomes possible to detect light in the sky, when the sun is 18° below the horizon. Nautical Dawn occurs at 12° below the horizon, when it becomes possible to see the horizon properly and distinguish some objects. Civil Dawn occurs when the sun is 6° below the horizon and there is enough light for activities to take place without artificial lighting.

Dusk occurs after sunset, once the top of the Sun has passed the horizon. As with dawn there is astronomical dusk, nautical dusk and civil dusk, occurring at 18°, 12° and 6° below the horizon respectively.

Twilight is the name given to the period between dawn and sunrise, or between sunset and dusk, when light is still visible in the sky due to sunlight scattering off the atmosphere. It can also be separated in astronomical, nautical and civil sections by how far below the horizon the Sun is.


Sunrise and sunset are the points at which the top edge of the Sun reaches the horizon; the only difference between them is the direction in which the Sun is moving at the time. It actually occurs when the top of the Sun is 0.57° below the horizon due to refraction of the Sun’s light by the atmosphere.

Talk Like a Physicist Day

Today, 3.14, is International Talk Like a Physicist Day. I talk like a physicist every day, because I’m a physicist. But if you’re not a physicist and you’d like to join in, then there are plenty of tips to be found at talklikeaphysicist.com.

Order of Magnitude: Use terms like “orders of magnitude” to describe significant differences of scale.

Negligible: When something is small, say it is “negligible” non-zero, but negligible.

Infinitesimal: If it is really really small, say it is infinitesimal.

Non-trivial: For a physicist, nothing is ever hard or difficult – it is always “non-trivial”.

First-order approximation : “That is only a first-order approximation to a good cup of coffee”, “The living room is clean. Well … at least to a first order approximation.”

Empirical Data: Any actual personal experience becomes “empirical data.” i.e. a burn on your hand is empirical data that the oven is hot.

Ground State: You’re not being lazy, you are in your ground state.

Extrapolation: A semi-educated guess is an extrapolation.

Ideal Case: You aren’t ignoring details, you are taking the ideal case.

Vanishingly small: A tiny amount is “vanishingly small” or “negligible.” Really small is “infinitesimal”.

Photons: It’s not light, they are photons. Turning on the lamp becomes “emitting photons”.

Exercise to Reader: The rest is history becomes “the rest is left as an exercise to the reader…”.

Not even wrong: Someone is making an argument using assumptions that are known to be wrong, or are making an argument that can’t be falsified.