Coordinates

The settings in this view let you select the coordinate system used to display the sky chart, and precisely center the chart on a particular set of coordinates. You can also set the field of view width and orientation, and display or hide the chart center coordinates and field of view while you are swiping or pinching the chart.

Coordinate System

This sets the coordinate system used by SkySafari. These are the options that you can choose here:

The horizon is only visible in the sky chart when using Horizon coordinates. In other coordinate systems, it would appear as a confusing distraction that tilts and move as the Earth rotates - so SkySafari hides it.

Chart Center Coordinates

Center Azm/RA/Lon: Sets the azimuth at the center of the sky chart. North is 0°, East is 90°, South is 180° and West is 270°. You may enter a new azimuth to precisely set the chart's center.

Center Alt/Dec/Lat: Sets the altitude at the center of the sky chart. At 0° the chart is centered on the horizon, at +90° it is centered directly overhead at the zenith, and at -90° it is centered directly under your feet.

When using Equatorial or Ecliptic coordinates, the chart center RA and Dec are always assumed to be for the precession epoch specified in the Precession settings.

When using Horizon coordinates, the chart center altitude is assumed to be apparent (i.e. it includes the effects of atmospheric refraction) if the Refraction option is turned on in the Precession settings. If this option is turned off, the chart center altitude is assumed to be the true (un-refracted) altitude.

Field of View

Field Width Angle: Sets the sky chart's field of view width angle using a slider control.

The largest field of view SkySafari can display is 180 degrees, letting you see the entire sky at once. As the field of view increases past 90 degrees, the horizon becomes curved, due to the distortion caused by projecting the entire celestial sphere onto the flat screen. In SkySafari Plus and Pro, the maximum field of view is 90 degrees when you are orbiting another solar system object. See the Orbit button Help for more information.

If you hold your mobile phone at arm's length, about 2 feet from your eyes, its 2-by-3 inch screen has an apparent size of 4.8 by 7.2 degrees. So, if you set the field of view width to 4.8 degrees (portrait mode) or 7.2 degrees (landscape mode), and hold your phone out at arm's length, the view on your phone should appear at the same scale as the real sky.

The smallest field of view SkySafari can display is 0.1 arcminutes, or 6 arcseconds - about the average size of the planet Mars as seen from Earth. One arcsecond is the best resolution a typical 8" backyard telescope can achieve under good observing conditions.

For comparison, the Sun and Moon appear about 1/2 degree (or 30 arcminutes) across. The smallest angle the unaided human eye can resolve is about 1/30th of a degree, or 2 arcminutes - about 1/15th the width of the full Moon. At its closest approach to Earth, the planet Venus appears about 1 arcminute across; Jupiter typically appears appears 45 arcseconds across.

Flip Horizontally: "On" flips the sky chart display horizontally to match the view in a telescope whose optical design results in a mirror-image view.

Flip Vertically: "On" flips the sky chart display vertically to match the view in a telescope whose optical design results in an upside-down view.