Sandburg Center for Sky Awareness
A Fairfax County Public Schools Planetarium

Equatorial Sundials and the Sun's Apparent Path Across the Sky

You can learn a lot more than the time of day by looking at an equatorial sundial!

Graphic courtesy The CoVis Project and The USA Today Weather Book by Jack Williams.

Summer Solstice

    An equatorial sundial is actually a reduced model of the Earth, similar to a globe with its upper and lower halves removed: the dial plate represents the plane of the Earth's Equator; the gnomon represents the Earth's axis of rotation. The upper dial face represents the Northern Hemisphere; the lower dial face represents the Southern Hemisphere. From the Spring Equinox to the Fall Equinox, when the Sun's apparent path across the sky is north of the Celestial Equator, the gnomon (or style) shadow falls on the upper dial face (as shown by the picture to the right, taken 23 June 2002).

    Every day, shadows are shortest at noon, and longest at sunrise & sunset. On June 21, noon shadows are the shortest of any day during the year (for northern mid-latitude locations), and vice-versa on December 21 (see solstice/equinox diagram). Due to the geometry of equatorial sundials, the gnomon shadow is the same length for the entire day (although its length varies from day-to-day according to the annual cycle of change in the declination of the Sun).

    Note that it is incorrect to say that the Summer Solstice is the "longest day" of the year. The fact of the matter is that the day is still 24 hours long--no longer than any other day of the year! However, it is correct to say that the number of hours of daylight is greatest (notice that the red dashed line is longest on the Summer Solstice diagram). Further, it is more correct to refer to the "Summer Solstice" as the "June Solstice," since the seasons are reversed in the Northern and Southern Hemispheres.

Spring or Fall Equinox

    On the day of the equinoxes, the Sun's apparent path across the sky (shown by the red dashed line on the Spring or Fall Equinox diagram) follows the Celestial Equator. Since the Celestial Equator is simply the projection of Earth's Equator onto the sky, you may also visualize the red dashed line as a segment of the outer edge of the dial plate of a very large equatorial sundial. The Sun is directly over the edge of the dial plate on the Spring or Fall Equinoxes (more correctly referred to as the March or September Equinoxes), therefore the gnomon (or style) of a properly oriented equatorial sundial will not cast a shadow on the dial plate.

Winter Solstice

    From the Fall Equinox to the Spring Equinox, when the Sun's apparent path across the sky is south of the Celestial Equator, the gnomon (or style) shadow falls on the lower dial face.

    Note that it is incorrect to say that the Winter Solstice (more correctly referred to as the December Solstice) is the "shortest day" of the year. It is correct to say that the number of hours of daylight is least (notice that the red dashed line is shortest on the Winter Solstice diagram).

© Copyright 2002-2008 Walter Sanford. All rights reserved.
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