Time: , northern hemisphere. RA at the top:
Time: , whole sky. The Ra at the center is (GCRS).
Time: , southern hemisphere. RA at the top:
This webpage uses the computer's clock to obtain the current time and plot star charts showing the positions of the Sun, Moon, planets and stars. The three star charts are based on the coordinate system whose center is at Earth's center and whose axes are oriented to the same directions as those of the International Celestial Reference System (ICRS). For simplicity, we will use GCRS to refer to this coordinate system. GCRS is practically the same as the equatorial coordinate system based on the mean equator and equinox of J2000.0. When the precession button is active, the Ra and Dec grids show the Ra and Dec lines with respect to the mean equator and equinox of date. When it is inactive, the grid lines are the Ra and Dec lines of the GCRS. The Ra labels in the polar charts always refer to the Ra of the GCRS.
The first chart shows the stars to the north of the GCRS equator. It is created using the stereographic projection with the GCRS south pole as the projection point. The third chart shows the stars to the south of the GCRS equator. It is created using the stereographic projection with the GCRS north pole as the projection point. The second chart shows the all-sky map using the Mollweide projection. Stereographic projection preserves angles but not areas, whereas Mollweide projection preserves areas but not angles.
Constellation labels, when active, are shown in the abbreviated form. The full names of the constellations are listed on this webpage.
Unlike the local star charts, the animation buttons apply to all 3 charts. When an animation plays, all 3 charts will be updated.
Stars in the charts can be clicked and a popup box will appear. The popup box shows further information about the star. When the time is between 3000 BCE and 3000 CE (CE = common era), positions of the stars with respect to J2000.0 mean equator and equinox are corrected for proper motion and annual parallax. The apparent position with respect to the true equator and equinox of date are corrected for precession, nutation, and aberration of light in addition to proper motion and parallax. Outside the time interval 3000 BCE - 3000 CE, only proper motion is included in the J2000.0 position and only precession and proper motion are included in the "of date" position. In the distant past and distant future, some nearby stars are seen to move far away from their present constellations. In that case, two constellations are given. The first one has "(2000)" added to it, indicating the present constellation. The second constellation has "(year number)" added to it, indicating the constellation in that particular year. The constellation is determined by the constellation boundaries established by the Belgian astronomer Eugène Delporte in 1930 on behalf of the International Astronomical Union (IAU).
Sun, Moon and planets are displayed on the charts by symbols indicated above. They can also be clicked and a popup box will appear. In the popup box, the term "elongation" is the angular separation of the planet and the Sun as seen from Earth; "heliocentric" refers to quantities measured from the Sun's center; "geocentric" refers to quantities measured from Earth's center.
Since the resolution of the star charts is low, the positions of the Sun and planets in the star charts are calculated using low-precision formulae. These formulae are only accurate between 3000 BCE and 3000 CE. Position of the Moon is calculated by a truncated ELP/MPP02 series. Positions stated in the popup box are calculated using more accurate formulae (VSOP87 for the Sun and planets and ELP/MPP02 for the Moon). In the popup box, when the time is between 3000 BCE and 3000 CE, the J2000.0 position only includes the light-time correction. The "of date" apparent position includes precession, nutation and aberration of light in addition to the light-time correction. Outside the time interval 3000 BCE and 3000 CE, only precession and light-time correction are included in the "of date" position.
When plotting the celestial objects on the charts, only the precession of the Earth's spin axis is included. The formulae used in the precession calculation are valid within 200,000 years before and after the years 2000.
Positions and apparent magnitudes of stars are adjusted based on their 3D motion in space, assuming they move in a straight line relative to the solar system barycenter. Since stars are very far away, only nearby stars show noticeable motion over a time span of millennia. The positions and 3D motion of the stars are obtained from the HYG 3.0 database. Milky Way is generated based on the Gaia data and the Milky Way boundary data provided by Jarmo Moilanen. The detail is explained here. Deep sky objects are not included.
The physics and mathematics involved in creating the star charts is explained briefly in this pdf document.