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.
This sets the coordinate system used by SkySafari. These are the options that you can choose here:
- Horizon - In the Horizon (or "Alt-Az") coordinate system, altitude is how high in the sky something is, and azimuth is the direction around the horizon. This system is used to show an object's position in the sky relative to your local horizon line.
- Equatorial - In the Equatorial (or "RA-Dec") coordinate system, RA stands for Right Ascension, and Dec stands for Declination. These coordinates are akin to longitude and latitude on the Earth. The Equatorial system is aligned with the Earth's equator and rotates with the Earth, so the coordinates of objects in the sky do not change as the Earth turns. Hence, Equatorial coordinates are commonly used with printed star atlases.
- Ecliptic - In the Ecliptic coordinate system, longitude and latitude in the sky are akin to longitude and latitude on the Earth. Ecliptic coordinates are defined by the plane of the Earth's orbit. The "equator" in this coordinate system is called the Ecliptic path or simply the Ecliptic. It is a great circle traced by the Sun as the Earth orbits around it over the course of a year. Most objects in the solar system orbit the Sun in nearly the same plane as the Earth, so they usually appear near the ecliptic in the sky. Ecliptic coordinates are the "natural" coordinates for the solar system, and are used extensively to describe the motion of planets, comets, and asteroids. Please Note: Ecliptic coordinates are only available in SkySafari Plus and Pro.
- Galactic - In the Galactic coordinate system, longitude and latitude in the sky are akin to longitude and latitude on the Earth. Galactic coordinates are defined by the plane of our Milky Way galaxy. Galactic longitude is measured westward along the galactic equator from 0° at the galactic center, located in the constellation Sagittarius, to 360°. Galactic latitude is measured from 0° on the galactic equator to +90° at the north galactic pole and -90° at the south galactic pole. Galactic coordinates are used most commonly for dynamical studies of stars, star clusters, galaxies, and other objects outside the solar system. Please Note: Galactic coordinates are only available in SkySafari Pro.
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.