The world in the palm of your hand

March 16, 2009. New version

Terra is an image of the Earth showing the positions of the Sun and Moon and the illumination by the Sun. You can see how these bodies stand in relation to each other at any location and any time. It is like a pocket calculator for Earth, Sun and Moon. It is practical, educational and beautiful.

Terra will show:

It will be useful to:

When the application starts it looks like this.

The top line shows the current date, time and time zone.

The second line shows the latitude and longitude of the current location, which is indicated by the white crosshairs and dot.

The line below the sphere shows the altitude and azimuth of the Sun and the length of day for the current location and time. A gold arrow points from the center in the direction of the sun, and a blinking gold "splat" shows the subsolar point, the place where the Sun is directly overhead. A blue arrow points toward the Moon, and a blinking blue disc marks the sublunar point.

The bottom line shows sidereal time, both Greenwich Mean Sidereal Time (GMST) and the sidereal time of the current location.

The terminator is the line between day and night. It is shown as a gray line circling the Earth. Behind the terminator are three more lines. They mark the commonly accepted twilights: "civil", "nautical" and "astronomical". They are respectively 6, 12 and 18 degrees past sunset.

NASA images of the nighttime Earth are used where correct when displaying the Sun or Moon information.

The controls:

The area over and around the sphere is a control. When you swipe your finger in any direction on this area the sphere will turn in that direction. "Inertia" will keep it spinning after you remove your finger. To stop it, touch the screen and hold your finger there for a half second or more, then remove it without swiping.

Press to call iPhone Location Services. It will use WiFi hotspots and the GPS to find the current location, and it will rotate the sphere to bring that location to the center. It will take a few seconds to acquire the information. Pressing it a second time might acquire a more accurate location.

This also resets the clock to current time.

Press to see information about the Sun and Moon.

The application starts by showing the Sun information. Press again to see information about the Moon. Its altitude, azimuth and current phase are shown. The sublunar point is a blinking blue disc. The blue arrow points from the center toward the Moon and is shown only when the Moon is visible from the center. The twilights are not shown, for simplicity.

Press again to see a plain sphere with the NASA daytime images and no Sun or Moon information. Another press returns to Sun mode.

Press to display lines of latitude and longitude. Press again to remove them. Longitude lines appear every 15 degrees. Latitude lines are 30 degrees apart. The equator and 0/180 longitude are in red. The 90/270 longitude is green. The rest are white.

Press to create a "pin" at the central location. When you rotate the sphere the bottom line will show the distance and compass bearing from the pin to the central point. Press again to remove the pin and information.

Moving through space:

The gray area at the top is also a control. When the application starts it is in "Space mode", indicated by the location coordinates in white and the date and time in black. In this mode swiping the surface will turn the sphere, and you will see the latitude and longitude change as it turns.

Moving through time:

Press the gray bar to change into "Time mode".

In Time mode, the date and time have white type, and the location is in black.

Swiping the screen in Time mode moves in time. An upward swipe moves ahead, downward moves back. You will see the date and time change as you do this. The rate of change is slowest when you swipe the right side of the screen, faster on the left. You can move by minutes or days at a swipe.

The Sun and Moon arrows, the twilights and the earth images change to show the passage of time. When the Sun rises or sets the transition is noted and shown on the line beneath the Alt/Az data. Similarly for the Moon when in that mode.

The time displayed at the top always refers to the iPhone's current time zone. The display will automatically show standard or daylight time as appropriate. If you want to use another time zone, use the iPhone Settings (General / Date & Time) to choose it. Press again to return to Space mode.

To return to the current time and location, press

To find the current position of the Sun or Moon:
Choose the location. For the current location, press

Press if necessary to display the Sun or Moon information.

For the Sun, the current altitude and azimuth and the length of the day are shown.
For the Moon, the current altitude and azimuth and phase are shown.

To find rising and setting times for Sun and Moon:

Choose the location and the mode, Sun or Moon.

Press the top button to go to time mode. The time shown will be the current time.

Swipe up to move forward, down to move back. When the gold arrow disappears a sunset is noted and displayed. When the arrow appears a sunrise is noted. The Moon information works the same way. You can proceed forward or backward as far as you wish to find rising and setting times for any date.

Rising and setting times are defined as the appearance or disappearance of the edge of the Sun or Moon, not the center. Refraction by the atmosphere makes things appear higher than they actually are. For the Moon, parallax must be corrected. Terra accounts for these effects. Because refraction varies with changes in the atmosphere, actual rising and setting times may be slightly different.

To find transit times for Sun and Moon:
A transit occurs when the center of the body crosses the zenith, i.e. it has azimuth 180 degrees. Move forward or back in time until the azimuth is 180. The time is shown in the top line.

To find times of twilights:
Move forward or back in time to bring a twilight line under the central point. The time display shows when that will occur at that location. The accepted twilights are "civil", "nautical" and "astronomical". They are respectively 6, 12 and 18 degrees past sunset. No adjustment for refraction, etc. is done for the twilight lines.

Details:

Terra has been tested and verified to work with iPhone OS 3.0.

Terra is demonstrated in a video on fora.tv.

Terra uses algorithms from the Astronomical Almanac (2009) for the positions of Sun and Moon. They will be within 0.3 degrees of the actual position.

Terra will run on any model of iPhone or iPod Touch. Units without a GPS may not be able to find the current location automatically.

Terra does not require a network connection.

Earth images and mapping courtesy of NASA.

Terra is a substantial upgrade of the iPhone application "Blue Marble". Owners of Blue Marble will get the upgrade at no charge.

The name "Blue Marble" is a trademark of Blue Marble Geographics.
 www.bluemarblegeo.com.