The night sky
Constellations, precession, what your latitude hides from you, and how much of it light pollution has already taken.
Constellations
3 guides filed under this heading
- The Sun is not in your constellation on your birthday
Precession has moved the equinox 28° since the zodiac was fixed, so the Sun is about a month behind the sign — and the signs are not constellations.
3 min read - Ophiuchus is on the ecliptic whether or not the zodiac admits it
The Sun spends about 18 days a year in Ophiuchus, more than in Scorpius. It is missing from the signs because the signs are calendar slots.
3 min read - Circumpolar stars: what your latitude puts out of reach
Declination decides whether a star ever sets. Above 90° minus your latitude it circles the pole forever; below the negative of it, it never rises.
3 min read
Observing
3 guides filed under this heading
- The Bortle scale: nine grades from pristine to unusable
A one-to-nine judgment of how dark your sky is, anchored to things you can actually see — the Milky Way, zodiacal light, the limiting magnitude.
3 min read - Why stars twinkle and planets do not
Scintillation is an angular-size effect. A star is a point of light, so the atmosphere can knock it about; a planet is a small disc, so it averages out.
3 min read - The green flash lasts about a second and is entirely predictable
Refraction splits the setting Sun into coloured images stacked vertically. For a moment only the green one is above the horizon.
3 min read
Other topics on Meridian
Every guide on this site is filed under one of these
The Moon
Phase, rise, set, distance and the illusions in between — what the Moon actually does, computed for a place and a date rather than described.
The Sun and the seasons
Day length, solar noon, twilight and the off-center clock: why the earliest sunset is not on the shortest day, and why the equinox is not twelve hours.
Eclipses and meteor showers
Shadow geometry and dust: which dates are real, where you have to stand, and the difference between a rate on paper and a rate in the sky.