SkyView software is used for determining the positions of the sun, moon, planets, stars and other astronomical bodies in the sky from the user’s location. It is designed to allow the observing parameters, such as date, time and location to be easily changed so that changes in an objects position in the sky can be studied.
The main aim to writing this software, was simply to understand the processes and computations involved to achieve this.
SkyView is a 64 bit application, and will soon be available for download from the Microsoft Store for Windows-10.
An MSI installation package of SkyView is also available upon request, for installation on Windows-7.
This current version is the first release and the accuracy of the computations seems to be quite good, but may be further refined later as required. Comparisons with other similar software generally show good agreement.
When SkyView first starts, the default view shows the Time menu on the left, a Sky Map plot in the centre and a listing the sun’s location on the right.
On the left side, there are 3 tabs that correspond to Parameters, Location and Time.
Parameters Tab. This gives some parameters controlling the plotting on the Sky Map plot. In the example, Orion is listed as a “highlighted constellation”, which is why it appears red in some of the following plots. The effect of changing the parameters is not realized until returning to either the Location or Time tabs.
Location Tab. Enter the latitude, longitude and elevation for your desired viewing location. The data for five locations can be saved in the lower table. Click “Sa” button to save the current parameters. Any existing parameters at that line will be over written. Click “Re” to restore the parameters from the selected line.
Time Tab. This is where most of the action takes place. By default, the time is set to the value PC time when SkyView was started. The Time Zone setting is taken from the PC clock and cannot by changed within this software.
The values of year, month, day, hour, minute and seconds can be incremented by clicking on the arrows either side of the value for these items. By default, they will increment in steps of 1 unit but the increment for each can be changed by using the corresponding increment slider.
The arrows next to the date/time values can held pressed down to show a continuous update. An application for this, for example, would be to set the current time to one hour after sunrise, then hold down the “day” arrow, to observe how the sun rise location moves daily.
The “Automatic update” button allows the software to run un-attended. There are 2 parameters associated with Automatic Update, and they are selected from the underlying drop-down menu’s. The first parameter is the “plot interval”, which is the time difference between the successive plots. The default is real-time, which means the plotting time is taken from the PC clock and updated very second. All the other options take the time from the manually selected values. The values of increment sliders are also considered, as the plot time is recomputed. The second parameter is the plotting refresh interval.
Sky Map plot
The Sky Map plot has a light blue background during the day time (when the sun is in the sky), and a light grey background during night time.
Clicking on an object on the Sky Map will do several things; (a) a red circle will appear around the object. Re-clicking on the object will remove the circle; (b) the listing will change to show information from the selected object, and (c) a Star Chart view will become centered on the selected object. If later the time is changed, the Star Chart will continue to stay centered on the selected object. The Star Chart will also follow the objects below the horizon. When region of the sky being viewed is below the horizon, the background becomes a light pink colour.
If the “shift key” is depressed when clicking on the Sky Map plot, then the Alt-Azim is selected instead of an object and the Star Chart will then stay locked onto the selected value of Alt-Azim, and will not follow any objects as the time is changed.
Clicking on the name of a star in the listing window will result in a small red circle to appear around that star. If the star was below the horizon when selected, then you’d have to advance the time until that star rises before it can be seen. Re-clicking on the star name will cause the red circle to disappear.
Checking the box labelled “Automatically flip to Star Chart”, will cause the Star Chart page to come up automatically if a star is selected either on the Star Map or from the selection list.
Clicking on the star name in the selection list whilst the “shift key” is depressed will rotate the sky until that star is at the meridian. That is, the time is changed so that the hour angle for that star is 360 degree. Be aware that there will be stars that will never be visible in the sky from your current viewing latitude (those that have a declination greater than negative value of your latitude). If you do want to view these stars, then select a latitude from the opposite hemisphere.
When clicking on a double star, the star selected may appear to be randomly one of either of the two components (assuming both components have a magnitude greater than the plot magnitudes). However, both stars will have slightly different locations in the catalogue, and clicking to one side of the star will consistently select the component that is to that side. For example, Centaurus is a double star and clicking to the Centaurus side of Centaurus will select the 1 component (Rigel Centaurus), and clicking on the other side will selected the component.
The Auto Rotate options shown on the Star Chart will rotate the Star Chart such that the horizon is at the bottom and the will be directly above the centre of the plot. The other angles are fixed relative to the N, E, S & W directions; for example, a value of 90 means that the Easterly horizon is in the same direction as the lower edge of the plot.
A Brief Summary of Computations
Computations involving the earth are:
- Latitude and longitude computations. These are restricted to WGS-84 system current version.
- Julian day and sidereal time computations, along with Delta T corrections.
- Values of the Obliquity of the Ecliptic, Precession and Nutation.
- Compute location relative to Geocentric Ecliptic coordinates.
- Parallax correction (Diurnal).
- Refraction correction.
Solar System, including the Sun
- Compute locations relative to Geocentric Equatorial coordinates using VSOP87C. Light time corrections are considered.
- Precession and Nutation corrections.
- Parallax correction (Diurnal).
- Refraction correction.
The raw star location data is taken from the Hipparcus catalogue and corrected to J2000 Epoch The raw data from the Hipparcus catalogue is as viewed from the Barycentre of the Solar System. This data is used on the main Star Map.
Corrections are then applied for, in the order shown
- Proper motion.
- Annual Aberration.
- Precession and nutation.
- Annual Parallax
- Atmospheric refraction.
In the listings, the data shown under “ICRS 2000” have proper motion correction applied, while the data shown under “ICRS Corrected” have after additional aberration, precession, nutation and parallax corrections. The data shown under “Apparent” have additional atmospheric refraction corrections applied.
Diurnal aberration and diurnal parallax corrections are not computed for stars, as these are extremely small.
The zoomed in Star Chart plot has Proper Motion and Precessions corrected applied. These two corrections are the more significant long term corrections and are the only two that can be noticed on the scale of that plot. If you focus in on a nearby star such as Sirius and scan across several 100’s of years, then the effect of Proper Motion can be observed by noting that Sirius will move off the constellation lines which are only corrected for Precession effects.
For a more insight into the vales of the stellar corrections, go to the main menu and click on “View->Proper Motion, Aberration and Parallax Plots for a Selected Star”.
Right Ascension and Parallax
The only location that the stated value of parallax of a star will be equal to the apparent variation in RA over the year, is when that object is on the celestial equator. As the declination of the object increases the variation in RA will be greater than the parallax of the object. This occurs since the RA lines all converge together at the poles. For example, Rigel Centaurus has a parallax of 0.7548 arcsec, but its variation in RA over a 6 month period is 1.5 arcsec.
This phenomenon exists for all the corrections applied, and an extreme examples occurs for Polaris at around the year 2100, when it is closest to the north celestial pole.
Accuracies and Validation
The computations are validated by comparison to worked examples in Meeus, and comparison to other software such as CalSky, TheSkyX and Occult4. Some minor differences observed can be attributed to using algorithms of different accuracy, differing databases of starting parameters and different definitions. It’s noted that even the different software give different values from each other. Further follow up and understanding on these issues is ongoing.
- Earth computations. Good.
- Moon. As mentioned in Astronomical Algorithms by Meeus on page 337, the accuracy of the position of the Moon is approximately 10” in longitude and 4” in latitude.
- Solar system. Using VSOP87C.
- Stars. When the Proper Motion, Aberration, Precession, Nutation and Parallax corrections are applied, the accuracies of the Right Ascension and Declinations should be much better than 1 second of an arc.
- Astronomical Algorithms, 2nd Edition. Jean Meeus.
- Planetary theories in rectangular and Spherical variables. VSOP 87 solutions. P. Bretagnon and G Francou.
- Multi-Language VSOP87 Source Code Generator Tool. http://neoprogrammics.com/
- Explanatory Supplement to the Astronomical Almanac. P. Kenneth Seiselmann
- CalSky website. www.calsky.com
- TheSkyX software. http://www.bisque.com/sc/pages/TheSkyX-Professional-Edition.aspx
- Occult4 software. http://www.lunar-occultations.com/iota/occult4.htm
The “Sky Map” plots. The stars on these plots have no corrections applied and are shown at J2000 Epoch.
The “Star Chart” plot. This objects on this plot are corrected for Proper Motion and Precession.