Where it sits
It's the gap between them, drawn. Abolish the uniform clock and the figure-eight vanishes.
Tell it apart
| Aspect | Analemma | Equation of time |
|---|---|---|
| Form | A curve | A number |
| Shows | Both components | East-west only |
| Used on | Sundials, globes | Timekeeping |
| Same data | Yes | Yes |
Two views of one thing. The equation of time is the analemma's east-west component expressed as a number — how many minutes solar noon runs ahead of or behind clock noon. The analemma is that number plotted against the Sun's declination.
What it doesn't mean
No — that's the ecliptic. The analemma shows something subtler: the accumulated error between solar time and clock time, plus the Sun's changing declination. The tilt and the elliptical orbit are there regardless; the uniform 24-hour clock is what makes them visible, by giving you a fixed hour to plot against.
It's lopsided. The two lobes are unequal because axial tilt and orbital eccentricity contribute different amounts and peak at different times of year. A perfectly circular orbit would give a symmetrical figure-8; a zero-tilt planet would give a straight line.
What it really means
Photograph the Sun from the same spot at exactly the same clock time every few days for a year, and it doesn't sit still. It traces a lopsided figure eight across the sky.
The analemma is a picture of a disagreement — between the time the Sun keeps and the time we keep. Two separate effects cause the drift. Earth's axial tilt means the Sun's apparent motion isn't parallel to the celestial equator, so its east-west progress speeds up and slows across the year. And Earth's elliptical orbit means the planet genuinely moves faster near perihelion and slower near aphelion. Neither effect alone would produce this shape: circular orbit gives a symmetrical figure-8, zero tilt gives a straight line. The lopsidedness is the two effects failing to peak at the same time of year.
Which means the analemma is not the Sun's path through the sky — that's the ecliptic, a different thing entirely. It's the accumulated error between solar time and clock time, plotted against the Sun's changing height. It exists only because we insist on a uniform 24-hour day. Abolish the fixed clock and the figure-eight disappears.
The top and bottom of the loop are the solstices; the equinoxes sit exactly halfway between them.
And the name is older than the curve. Greek análēmma meant a prop or support of any kind, and already carried a sundial sense of its own; Latin kept only that one. The word was borrowed from the hardware of measuring time long before it named the shape that measuring time produces.
Where it comes from
Greek — prop, support of any kind
Greek analemma is a prop or support of any kind, and LSJ also records a sundial sense; Latin keeps only the sundial. English takes it in the 1650s for a projection of the sphere; the figure-of-eight sense arrives only in 1832.
- geometry
Produced by two combined effects: the tilt of Earth's rotational axis relative to its orbital plane, and the elliptical shape of that orbit.
- solstice markers
The top and bottom of the figure-eight are the solstices — the northern and southern limits of the Sun's sky motion; the equinoxes sit at the vertical midpoint, declination 0° — not at the loop's crossing point, which falls in mid-April and late August.
In real life
- Photograph the Sun from the same window at noon by the clock once a week for a year, and the pictures stack into a lopsided figure eight.
- Many older globes print a figure-eight scale in an empty stretch of ocean, showing for each date how far north or south of the equator the Sun stands overhead.
- Seen from the northern hemisphere, the top of the loop is the June solstice and the bottom is the December one.
Questions
Q01What is an analemma?
The figure-eight curve you get by marking the Sun's position at the same clock time each day across a whole year. Its width comes from solar time and clock time drifting apart and reconverging across the year; its height comes from Earth's 23.4-degree axial tilt carrying the Sun north and south.
Q02What causes the figure-eight shape?
Two things together: Earth's axial tilt relative to its orbital plane, and the elliptical shape of its orbit. Remove the eccentricity and you'd get a symmetrical figure-8; remove the tilt and you'd get a straight line.
Q03What do the top and bottom represent?
The solstices — the northern and southern extremes of the Sun's motion. The equinoxes sit exactly halfway between them on the curve.
Sources
- Sunrise Analemma — NASA Astronomy Picture of the Day.
- Build your own analemma — arXiv.
- analemma (etymology) — Wiktionary.