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Locations and Times Rise and Set Times Equatorial Chart

Local Star Charts

Location 1:       Longitude:       Latitude:

Local Time: ,    Sidereal Time:

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Location 2:       Longitude:       Latitude:

Local Time: ,    Sidereal Time:

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Constellation List

Description

This webpage uses the computer's clock to obtain the current local time and then uses it to calculate the local sidereal times and plot star charts on two locations. The two default locations are at longitude 88.2434°W, latitude 40.1164°N (Champaign, IL, USA) and at longitude 88.2434°W, latitude 30°S. Sidereal times and star charts at other locations and times can be obtained by clicking the  Locations and Times  button at the top of the page and filling in the form.

Constellation labels, when active, are shown in the abbreviated form. The full names of the constellations are listed on this webpage.

When the Day/Night button is active, the background color of the star chart is determined by the Sun's position in the chart: light purple when the Sun is above the horizon, gradually changes to black when the Sun is below the horizon and black when the Sun is 18° below the horizon.

The star charts are created using the stereographic projection with the nadir as the projection point. Stereographic projection is commonly used in sky maps. The mapping is conformal and shapes are preserved over a small area. However, the mapping does not preserve area. For example, a constellation is about twice as big when it is near the horizon than when it is near the zenith. This effect is quite noticeable in animations showing the diurnal motion of the sky. This feature might not be as bad, since constellations do appear bigger when they are close to the horizon because of the Moon illusion. However, it should be noted that stereographic projection is not designed to model the Moon illusion.

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 BC and 3000 AD, 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 BC - 3000 AD, 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 quantity measured from Earth's center; "topocentric" refers to quantities measured from the observer's location. For example, the geocentric distance of the Moon is the distance between Moon's center and Earth's center. Topocentric distance of the Moon is the distance between the Moon's center and the observer. The difference between the geocentric and topocentric position is called the diurnal parallax or geocentric parallax. This effect is particularly important for the Moon, which can be as large as 1°, but small for the Sun (≤ 8.8") and planets.

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 BC and 3000 AD. 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 BC and 3000 AD, the J2000.0 positions only include 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 BC and 3000 AD, 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 stars are obtained from the HYG 3.0 database. The milky way boundary is based on the data Milky Way and Magellanic Clouds provided by Jarmo Moilanen. Deep sky objects are not included.

The physics and mathematics involved in creating the star charts is explained briefly in this pdf document.