Since last post I’ve had to back off a tad from new developments with Star Atlas. Reason:- My Sidereal Clock wasn’t keeping proper time! Now, it wasn’t just out a constant few seconds or even a constant few minutes. No, my Sidereal Time seemed to be further and further off the mark the longer my program ran! Not a great amount but enough to make the star positions inaccurate after just a few minutes.

Now, I’m glad you’re asking, “What is Sidereal Time and why is it important?”. Sidereal Time is time measured by the stars. The time we use everyday (the time kept by our watches) is called Local Mean Time (LMT) and is derived from Local Apparent Time and… well, its all based on the position of the Sun. Sidereal Time, on the other hand is time based on the position of the stars. If you were to look at a bright star in your night sky (say Alpha Orionis or Betelgeuse in Orion) and record it’s altitude and azimuth (position in the sky) at a given time you would find that on the next evening the star would reach that same point 4 minutes earlier (by your LMT or your “watch” time). So, your Sidereal Clock is running approximately four minutes faster than your LMT clock.

Now, to calculate the instantaneous position of a star in your night sky you need to know the Right Ascension and Declination (sort of the Lat and Long) of the star, your Longitude, your Latitude, and your Sidereal Time (derived from your Coordinated Universal Time (UTC) and your Julian Date). In computer speak, here are the formulas:-

Julian Date = 367 * CurrentUTC.Year – Math.Floor(7 * (CurrentUTC.Year + Math.Floor((CurrentUTC.Month + 9) / 12)) / 4) + Math.Floor(275 * CurrentUTC.Month / 9) + CurrentUTC.Day – 730531.5 + (CurrentUTC.Hour + CurrentUTC.Minute / 60 + CurrentUTC.Second / 3600) / 24

Mean Sidereal Time = 280.46061837 + 360.98564736629 * Julian Date + Longitude

(Math.Floor simply means take the nearest integer (non-decimal) value.

So, you can see why I might have goofed a bit on originally coding them. Anyway, all is well now; well, almost. This particular version of the Mean Sidereal Time formulas will keep my Sidereal Clock accurate to within 0.001 seconds for the next 10 years but will only be accurate to 0.1 seconds a hundred years from now. Such is life; more accurate formulas are needed. Stay tuned.

So, after fixing up the Sidereal Time issue I added one new feature and one new improvement. I redesigned my splash screen (the screen that displays on program start) to look a tad niftier and I coded the feature that allows the program to be used from anyplace on the face of the earth! Yep, you can now select one of 850 cities (facilities for user defined locations and locations read from a GPS will be added soon) and you will see the night sky configuration for the current time in that city.

The latest updates added to my Star Atlas Planetarium sofware:-

User Selectable Date and Time

The time and date can now be changed by the user. Up until now the application would read the system time and display the position of the stars, in real time, based on the time offered by the system clock. The user can now select any date/time combination from January 1, 1753 to December 31, 9998. The caveat being, of course that for dates far from the epoch date (year 2000) of the stars in the star catalog the less accurate the star positions are. But, for reasonable deviations from the year 2000 this is not a problem. Future versions of Star Atlas will recalculate the epoch date if the deviation from the default epoch is too great. Once the user date has been selected the sky will, of course, change in real time as if it were actually that day and time.

The usefulness of this feature is that the user can now choose to see the sky at the time of their choosing. Its really cool, for e.g., to look at the sky on the date and time you were born, or to create a star chart based on some evening next month, etc, etc.

Sky Animations (Time Warping)

This feature is really cool. From the currently selected date and time the user can choose to advance the sky any number or combination of years, months, days, hours, and minutes. The refresh rate of the display is selectable from a fast update setting of 10 milliseconds to incremental update settings from 1 to 10 seconds. The upshot is that the user can advance (or reverse) time as the need arises. So, for example, the user may be viewing the sky on January 22, 2008, at 5:00 PM but might be interested in finding out when the belt stars in Orion cross the southern meridian. The user would simply configure and run an animation to advance the sky in say 4 minute steps every 1 second and then watch for the time when Orion’s belt crosses the meridian. Running the animation would cause the sky display to change every second with the time (and the star positions) advancing by 4 minutes with each display refresh. Its hard to describe in words but essentially the operation gives an animated view of the stars moving through the sky in “sped up” time.

I tried to make some movies of these new features but, alas, YouTube strips out too much detail and makes the videos to small for full screen views of Star Atlas to be meaningful. You’ll just have to wait for the first beta release of Star Atlas to experience the joy and elation brought on by Sky Animations!

Clear Skies!

Things are moving along nicely with my planetarium software project. Some improvements since last post:-

• Constellation names are now being displayed and can be turned on and off.
• Constellation lines can be turned on or off.
• The local horizon can be turned on or off and can be displayed separately from the Alt/Az grid.
• The local meridian can be turned on or off (with 10 degree tick marks) and can be displayed separately from the Alt/Az grid.
• Zoom in and zoom out are enabled.

Following is a screen shot of Star Atlas looking south from Halifax at approx. 1:45 AM on December 29. The Local horizon (curved green line at the bottom) and the local meridian (vertical line, very faint in the screen capture, with the tick marks) are visible.

A beta release may soon be available for friends and family!

(Click for a larger image)

Well, I’ve been away from my blogs for a bit. Reason:- I’m writing a planetarium application and, bloody hell, its taken over my life!! The following is a picture of the opening screen. What are you seeing? Well, you’re looking West into the night sky from Halifax, NS. Above the green line are stars that are visible above our horizon (at the moment this screen capture was taken) and below the line are stars below our horizon and, therefore, not visible from our location at the moment. The green lines, including the dark green horizon line are lines of Altitude and Azimuth (ALT/AZ); they indicate a stars position above the horizon and its angular distance around the horizon from North. The white lines indicate Right Ascension and Declination (RA/DEC) lines. RA/DEC is like Latitude and Longitude for stars. The bright white RA/DEC lines indicate zero hours RA (the one going up at 45 degrees in this image) and(the smiley faced one) zero degrees DEC. The ALT/AZ position of a star will change in real time whereas the RA/DEC position of a star will not change appreciably over the span of human lifetimes.

So far I have limited features enabled. I can turn any of the grids (or constellation lines) on or off, display the stars in real time (their positions will change with respect to the local horizon in time with the computer system clock), I can choose to look North, South, East, or West or scroll the globe 360 degrees in any direction or even display the Zenith (the point directly overhead) in the center of the globe. This is a useful feature in that it puts the local horizon around the perimeter of the globe and effectively creates an instantaneous star map of the stars and constellations in the night (or daytime) sky for any instant of time.

There’s still loads of work to do on this project. When completed, the user will be able to zoom in on any area of the sky, will be able to track planets, comets, satellites, view the positions of galaxies, globular clusters, planetary nebulae, etc, etc. At the moment the application uses only the Yale Bright Star Catalog (9,110 objects) as its data source but I have plans of adding at least the 2.5 million object Tycho Star Catalog. Oh, of course, it will eventually display the stars as they would look from any point on the Earth’s surface (I may even enable GPS capabilities to make this feature more accurate). So far its been engrossing, educational, and just plain lots of fun!

(Click the image for a larger view. It looks infinitely better when its not a JPG image!!)

Solar System in November has been updated. Click on “WhatsUp” under Categories in the left panel to see the entire post.