About


Special Credit

The original scripts were developed by Yuk Tung Liu available on GitHub


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 in Brunei Darussalam at Bandar Seri Begawan (Location 1) and at Kuala Belait (Location 2). 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.

The default settings can now be changed by filling in a 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 azimuth-at-the-top parameter is used to rotate the star chart, which is convenient for laptop and desktop users since the device's screens can't be rotated easily. 0° means north is at the top, south at the bottom (useful for looking at stars near the southern horizon); 90° means east is at the top, west at the bottom (useful for looking at stars near the western horizon); 180° means south is at the top, north at the bottom; 270° means west is at the top, east at the bottom.

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 and so the distortion should not be regarded as a faithful representation of human perception.

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 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 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 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 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 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.



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