Follow the subsolar point

Intro

A place that currently has the sun directly above is called the subsolar point. This always happens at the place’s solar noon. Most often around 12 o’clock since a majority of countries in the tropics do not observe daylight savings 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 set the map grid to your liking. Reasonable defaults are used if you leave it on Auto, it follows the zoom level. In the dropdown list you can see the interval in degrees and the time it takes the sun to move betwwen two grid lines. The sun moves 15 degrees in an hour.

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].

Thoughts?

I rewrote this page recently, hopefully improving it. Likely a few unsquashed bugs remain, time will tell. If anything has stopped working, use the contact page and let me know. If you can’t stand the new look, the old page is archived here.

🌞 May the sun always shine on you!