Daylight chart

How to read the daylight chart

This chart turns a full year of sunrise and sunset calculations into one compact daylight map. Each vertical slice is one local calendar day for the selected city. The shaded band starts at sunrise and ends at sunset, so a wider band means more usable daylight. The top of the plot is midnight, the middle is noon, and the bottom is the next midnight. Hover the SVG to inspect the exact daylight length for a date.

The solstice markers come from the longest and shortest daylight days found in the computed year, rather than fixed calendar dates. Equinox markers are chosen from the days closest to twelve hours of daylight around the March and September transition windows, with labels adjusted for the city's hemisphere. At high latitudes the sun can stay above or below the sunrise horizon all day. Those polar days are drawn as full-height daylight columns, while polar-night days are drawn as a thin baseline so the missing sunrise and sunset remain visible.

Daylight swing by latitude

How much the band widens and narrows across the year depends almost entirely on distance from the equator. A few reference latitudes show the scale of the effect; southern latitudes mirror these figures with the June and December dates swapped.

Switching between a near-equator city and one above 60 degrees makes this vivid: the tropical chart is an almost flat ribbon, while the high-latitude chart bulges into a wide summer lens and pinches to a winter sliver.

How each day's band is calculated

Every column starts as a fresh sunrise and sunset calculation for that date at the city's coordinates. The sun's elevation is sampled through the local day, and the moments it crosses -0.833 degrees, the standard horizon allowing for atmospheric refraction and the visible width of the solar disc, are refined to the nearest minute. Day length is the gap between those two crossings. Nothing is interpolated from neighbouring days, which is why one-off quirks such as leap days appear exactly where they happen.

The four season markers are likewise derived from the computed year rather than fixed calendar dates. Solstice labels attach to the widest and narrowest bands the calculation finds, and equinox labels attach to the days nearest twelve hours of daylight inside the March and September windows. For a city south of the equator, the labels flip so the widest band reads as the summer solstice in December.

Why the band edges jump an hour twice a year

In cities that observe daylight saving time, the sunrise and sunset edges take a sudden one-hour step in spring and autumn while the width of the band glides on smoothly. That is not an error. The edges are plotted in local clock time, and the clock itself moves; the amount of daylight does not. The step is a compact picture of what a clock change actually does: it relabels an hour of morning light as evening light, or the reverse. Cities that skip seasonal clock changes show smooth edges all year round.

Daylight chart questions