Follow the subsolar point

Intro

A place where the sun is directly overhead is called the subsolar point. This occurs at the location’s solar noon, which is typically around 12 o’clock. This is because most countries in the tropics do not observe daylight saving time.

The sun can be at its zenith anywhere between the tropics of Cancer and Capricorn. That is the portion of the map that has no shade. Locations closer to the edges of the tropical zone have more days with zenith events, this is because the sun’s ”vertical speed” slows down close to the solstices. Imagine a sine curve which is flat at the top/bottom.

The yellow circle (now at × )

The circle shows the current subsolar point. Its position updates auto­matically and moves from right to left along the yellow line.

The yellow line

A yellow stripe across the map shows where the sun passes today ( UTC). It is quite accurate, probably down to a few hundred meters or so. Zoom in to get a preciser view. Click anywhere on the yellow line to see the sun’s arrival time.

This line is valid for the date just mentioned and updates by itself at midnight (UTC). Due to the equation of time, the line may be a few mm off ±15 minutes around midnight.

The map grid

You can customize the map grid to your preferences. If you leave it on Auto, it uses reasonable defaults and follows the zoom level. The interval in degrees and the time it takes the sun to pass between two grid lines are displayed in the dropdown list. In an hour, the sun moves roughly 15 degrees.

Automatic display updates

If set to Auto, the update rate varies from three per minute at lowest zoom, to once per second at max zoom. Auto should be good for most cases but you can change it to a fixed value.

You can tick the Center box and have the map always centered on the moving subsolar point. Use it when you are zoomed in and don’t want to slide the map by hand.

The moon

This is a solar page, but the moon’s current position and path are also shown. If this bothers you, just turn it off. No forecasts possible. The moon moves slower sideways than Sun, but much faster up/down. The turning points often go outside the shaded areas of the map.

Forecast for any place

Just click on the map and the table will be populated with one year’s worth of solar zenith events.

The sun’s height (altitude) is measured in degrees above the horizon, from 0° to 90°. It is unlikely the altitude will be exactly 90.0°, so any altitude greater than 89.5° will be considered ”in zenith” and shown in the table.

As a default, the time in the forecast table is shown in UTC. You can switch to your local timezone if you desire. See the following section on timing.

Clock & timing

After the page is loaded, position updates run in your own browser. It uses dead reckoning and absolutely requires that the clock on your computer be accurately set. A 1 minute clock error displaces the subsolar marker with more than 25 km.

”Accurately set” also means using the correct timezone: The browser has to translate your local time to UTC and happily uses whatever timezone you have set. If you see weird timing errors of whole hours, it is likely a timezone issue. Your local timezone appears to be [not found, actually].

🌞 May the sun always shine on you!

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An older, unmaintained version of the page is archived here.