The EarthDial Project • FCPS/NOVAC EarthDial Webcam
TJHSST Planetarium, Fairfax County, VA
38° 49' N latitude, 77° 12' W longitude

Composite Xearth/ED-7 24-hour Time-Lapse Animation


From left-to-right, a composite three-panel time-lapse animation (above) showing:
  1. a rotating Earth, centered on the subsolar point (marked by red crosshairs);
  2. 24-hours of ED-7 images from yesterday, beginning after midnight;
  3. and a fixed Earth, centered on 10°N, 77°W, a point south of ED-7. The approximate location of ED-7 (~39°N, 77°W) is marked by red crosshairs.
In the "rotating Earth" view (left panel, above), the Sun shines directly on the subsolar point (marked by red crosshairs) and the globe appears to spin underneath the subsolar point. The day side of the Earth faces toward the viewer; the night side faces away from the viewer. In contrast with the "fixed Earth" view, the "rotating Earth" view is the better model of the actual Sun-Earth system.

In the "fixed Earth" view (right panel, above), the Sun is like a flashlight that revolves around the Earth, causing the terminator (the line that divides day & night) to appear to move from east to west (right to left). Using this analogy, the subsolar point is the position on Earth where the flashlight beam shines directly.

Was it cloudy yesterday? See also a one-month archive of past composite time-lapse animations (filenames only), including a permanent archive of files from the 1st and 15th of each month (filenames only).

Observing Daily & Annual Cycles of the Sun

Two motions--the rotation of the Earth around its axis, and the revolution of the Earth around the Sun--cause daily- and annual cycles in the Sun's apparent path across the sky that can be observed indirectly using a horizontal sundial such as the FCPS/NOVAC EarthDial (ED-7).

Viewed from above the North Pole, the Earth appears to rotate counterclockwise. Shadows fall in the opposite direction as the Sun. Therefore the shadow of the gnomon/nodus appears to move clockwise around the dial face in a daily cycle, as shown in the 24-hour time-lapse animation of ED-7 imagery (shown center panel, above).

On Earth, the subsolar point migrates north-south between the Tropics of Cancer & Capricorn in an annual cycle. In the "rotating Earth" view, the position of the crosshairs (representing the subsolar point) is fixed in the exact center of the image; as the subsolar point migrates north-south during the course of a year, the globe will appear to gently "rock" back & forth underneath the crosshairs. See the following three-panel time series of still images, as well as a time-lapse animation (~1.5 MB) showing the position of the subsolar point at 12 noon EST each day (1:00 p.m. EDT) for one year. If you watch the time-lapse animation carefully, then you should notice that the crosshairs trace the analemma (the odd-looking figure eight that appears on many globes).

DEC Solstice MAR & SEP Equinoxes JUN Solstice

On the day of the December Solstice (left panel, above), the Sun reaches the southernmost line of latitude (23.5°S, the Tropic of Capricorn) where the midday Sun is at the zenith. At northern mid-latitude locations, the midday Sun reaches its minimum altitude (annually) and the duration of insolation (number of hours of daylight) is least. For example, in Washington, D.C. the midday Sun will reach an altitude of 26.5 degrees above the horizon and there will be 9h26m of daylight. Also, the Sun rises farthest south of east and sets farthest south of west than at any other time of year.

On the day of the March- and September Equinoxes (center panel, above), the Sun crosses the plane of the Earth's Equator and day and night are of nearly equal length. The Sun rises exactly due east and sets due west.

On the day of the June Solstice (right panel, above), the Sun reaches the northernmost line of latitude (23.5°N, the Tropic of Cancer) where the midday Sun is at the zenith. At northern mid-latitude locations, the midday Sun reaches its maximum altitude (annually) and the duration of insolation (number of hours of daylight) is greatest. For example, in Washington, D.C. the midday Sun will reach an altitude of 74.5 degrees above the horizon and there will be nearly 15 hours of daylight (14h54m). Also, the Sun rises farthest to the north of east and sets farthest to the north of west than at any other time of year.

The FCPS/NOVAC EarthDial features three east-west lines showing the Sun's declination, similar in appearance to the stylized globe graphic, shown right. From top-to-bottom on the dial face, these "date curves" represent the December Solstice, the March- & September Equinoxes, and the June Solstice, respectively. In an annual cycle, the shadow of the nodus will appear to migrate up-down (north-south) among the declination lines (date curves). Tell the approximate time of year by comparing the position of the center of the shadow of the nodus to the (solar) declination lines. Note: On the FCPS/NOVAC EarthDial, the top and bottom declination lines are located in reverse order from the equivalent lines of latitude on Earth. The reason is simple: When the Sun's midday altitude is lowest (DEC Solstice), noon shadows are longest; when its midday altitude is highest (JUN Solstice), noon shadows are shortest. Remember, shadows fall in the opposite direction as the Sun!

See Earth's Seasons - Equinoxes and Solstices, a four-panel one-year time series of one-day time-lapse animations (~6.5 MB) created from ED-7 still images captured on the equinoxes and solstices in 2004: 20 March; 21 June; 22 September; and 21 December. See also a time-lapse animation (~20 MB, 320x240 pixels) of ED-7 still images showing the annual north-south migration of the nodus shadow at 12 noon Eastern Standard Time (1 p.m. EDT). The animation is updated once each day at ~1:10 p.m. ET: the first frame is always 12 noon on 15 February 2004; the last frame is either 12 noon on the previous day (before 1:10 p.m. ET) or 12 noon on the current day. The "movie" looks a little "jumpy" as a result of changes in the camera position and image text caption. See also a running archive of ED-7 still images from 12 noon ET each day since 15 February 2004 (filenames only).

How do we do it? See Technical Information re: ED-7 Archiving, a report by Phillip Wherry, ED-7 Technical Consultant.

© Copyright 2004-2012 Walter Sanford. All rights reserved.

FCPS/NOVAC EarthDial Webcam | ED-7 Team | The EarthDial Project