Day and Night Across the Earth  ·  September 30, 2020, 16:20 GMT
Right now: Moon at 24°44' Pisces, Sun at 7°58' Libra

Day and Night Across the Earth

Day and Night Across the Earth

This page provides a set of five synthetic images of the Earth's surface, each from a different perspective, that show the areas in sunlight (day) and darkness (night) at the specified date and time. Simply fill in the form below and click on the "Show Earth Views" button at the end of the form. The images can be produced for any date and time from year 1700 through year 2030.

It takes 15-60 seconds of CPU time on our computer to produce the images, depending on how busy it is and whether you select grayscale or color. Color output takes about three times longer to generate than grayscale. However, if the images have previously been created for the date and time you specify, they will appear immediately. Five images are produced and arranged on the output page. Each is only 10-40 kbytes, so bandwidth is usually not the major factor for wait time.

Date and time:

Use UT (Universal Time).

Year:   Month:   Day:

Hour:   Minute:     UT

Grayscale    Color (takes much longer)

    (Please be patient.)

Notes on the Images:

A set of five images is produced: one large cylindrical projection of the whole Earth and four smaller orthographic projections. Any of the images can be saved separately by right-clicking near the center. On each one, dotted lines of longitude and latitude are shown every 30 degrees, beginning at 0 degrees longitude and latitude.

The whole-Earth map is constructed so that the sunlit area is always in the middle. The point on the Earth's surface where the Sun is at the zenith (directly overhead) is indicated by the Sun symbol near the center of the map. The four orthographic projections (views of the Earth from infinity) are centered on the north pole, the south pole, the point where the Sun is at the zenith, and the point where the Moon is at the zenith. The latter two views thus show the Earth as it would be seen from the directions of the Sun and Moon, and are labeled "Sun View" and "Moon View", respectively. (To be strictly correct, straight lines from the center of the Earth to the Sun and Moon at the specified time would not pass exactly through the points used, but the difference is quite small and below the resolution of the images.) In these last two images, a horizontal line would be parallel to the plane of the Earth's orbit around the Sun (the direction to the north ecliptic pole is up).

If you advance the requested time by an hour or more, you can see the continents shift to the right in the whole-Earth map and the Sun and Moon views, due to the Earth's rotation. The north pole view would show the Earth rotating counterclockwise and the south pole view would show the Earth rotating clockwise.

The dividing line between day and night (which is fuzzy) is called the terminator. On the whole-Earth map, the Sun is rising at places along the terminator where the sunlit part of the Earth is to the right, and setting at places along the terminator where the sunlit part of the Earth is to the left. On this map, in the polar regions, the terminator is horizontal (along a line of constant latitude) or nearly so for part of its length; at such places, the Sun is "skimming" the northern or southern horizon. At any place along the terminator, the local direction of the Sun on the horizon is perpendicular to the terminator, toward the point on the Earth where the Sun is at the zenith.

The shape of the terminator curve on the whole-Earth map changes significantly over the course of a year, and it is instructive to compare the terminator at the equinoxes (on or about March 21 and September 21) with the same curve at the solstices (on or about June 21 and December 21). Because our atmosphere bends sunlight about a half-degree (60 km) into the area of the Earth that would otherwise be dark, the sunlit part of the Earth is slightly more than half of the entire surface (there is actually more than 12 hours of daylight at the equinoxes). The shape of the terminator shown on these images is adjusted for this effect. In particular, at the dates of the equinoxes, you can see that both the north and south poles are in sunlight. In fact, viewed from either pole at the exact time of the equinox, the entire disk of the Sun is clear of the horizon.

The Moon view shows the area of the Earth where the Moon is above the horizon. The Moon view also shows the phase of the Earth as seen from the Moon; it is opposite the phase of the Moon as seen from the Earth. For example, a gibbous Moon corresponds to a crescent Earth, a new Moon corresponds to a full Earth, etc. (See, for example, today's phase of the Moon for a comparison.)

In constructing these images, the geometry of the Earth, Sun, and Moon is computed from the data in the Jet Propulsion Laboratory DE405/LE405 planetary and lunar ephemeris. The cloudless Earth map that is used is a processed version of a mosaic produced at NASA's Earth Observatory from images taken by the Terra satellite.

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