SkyView is used for determining the positions of the sun, moon, planets, stars and other astronomical bodies in the sky from an observer's location. It is designed to allow the observing parameters, such as date, time and observer location be easily changed so that changes in an objects position in the sky can be studied.
A map of the entire sky is given showing the stars, planets and other bodies that are currently above the horizon and observable. A zoomed-in star chart that is centered on a specific body is also produced, and is independent of whether the body is above the horizon or not.
The computations can be done over a date range from 4713 BCE to 9999 AD.
Other features include:
- computing the rise and set times of astronomical bodies.
- predicting the phases of the moon, and the dates of lunar and solar eclipses.
- computing the date and time of the Solstices and Equinoxes.
- plots of the Solar Analemma and Equation of Time.
- almanacs giving the annual rise,set and meridian transit times for the sun and the moon.
- predicting passes of the International Space Station.
- showing the sky as it would be observed from the surface of another planet.
- computing "Mars Time".
- plotting the corrections that are required for Obliquity, Nutation and Precession.
- plotting the corrections that are applied to the RA and Declination of the stars.
- showing the current location in the sky of the GPS, Glonass and Iridium satellites.
- converting Latitude and Longitude to UTM and vice versa (WGS-84 only).
- local times are referenced to daylight saving time when applicable.
- historical time-zone settings can be applied for any location on Earth.
- show the date in both the Gregorian and Julian calendars
- computing the date in the Islamic and Jewish calendars.
This application is free to download, but some of the features listed above require an Extra Features license that is available as an In-App purchase from the Microsoft Store. A 7 day free trial license or a purchasable lifetime license are available.
SkyView is a 64 bit application, and is available for download from the Microsoft Store for Windows-10.
Go to https://www.microsoft.com/store/apps/9PHZWXDT4B79 to download and install. Please give the software a rating, near the bottom of the Windows Store download page.
The main aim to writing this software, was simply to understand the processes and computations involved in predicting the locations of the observable astronomical bodies in the sky. An additional objective to achieve good accuracy in the computations. This has been an ongoing process and comparisons with other similar software and almanacs generally show good agreement.
When SkyView first starts, the default view shows the Time menu on the left, a Sky Map plot in the center and a listing the sun’s location on the right.
The observer location will default to Perth when it is first started up after installing. A new observer location can be selected by clicking on the "Obs Location" button. This will bring up an "Observer Location" window. To change the observer location. either click on the map, or type in new values of the latitude and longitude.
A list of changeable parameters can be found under Views->Parameters.
The view of the sky that is displayed is that which would viewed from the currently selected Observer Location at the date and time that has been selected. The Time Zone setting is defined by the Observer location.
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.
Sky Chart plot
The Sky Chart 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. The Sky Chart can see the entire sky and when the view-able area is below the horizon, the background is colored light pink. There is an option to turn off the background coloring.
A single click on an object will cause the chart center on the object clicked. Note that only a single click is required.
If the SHIFT key is depressed whilst scrolling over the Sky Chart, then a pop-up with red text text will appear giving information on the star nearest to the mouse cursor.
A small scroll bar in the orientation box is used to rotate the Sky Chart when a fixed angle value has been selected.
Object Coordinate Data
The coordinates of the Sun, Moon and Planets are initially computed in geocentric ecliptic coordinates. These are transformed to geocentric equatorial coordinates, and finally to topocentric equatorial and horizontal coordinates.
Topocentric coordinates are corrected to the observers location on the Earth's surface. The topocentric correction is largest for nearby objects such as the moon, and very small for the far planets.
The "apparent" coordinate have atmospheric refraction corrections applied, which will show a notable difference only when the object is very close to the horizon. When an object is below the horizon, refraction correction has no meaning and is not applied.
Examples are shown below for Jupiter on 23rd and 24th Feb-2019. The first list is at 15:55 local time when Jupiter is below the horizon and is therefore un-observable, so the topocentric coordinates are shown in gray font. Later at 2:56 on the next day, Jupiter is above the horizon and the topocentric coordinates are in black font.
Stars coordinates are derived from a database of Equatorial coordinates. Different types of corrections are required and applied for objects outside of the solar system, and these are discussed later.
Solar plots and data
Clicking on the "Sun" button in the top menu brings up a window showing the Sunrise and Sunset times.
There are 4 other displays that are accessible from this window that include Solstice/Equinox date and times for the current year, Solar Analemma at the current location, the Equation of time plot and an Almanac listing of the sunrise, sunset and meridian transit times for the current year.
Lunar plots and data
Clicking on the "Moon" button in the top menu brings up a window showing the Moonrise and Moonset times.
There are 3 other displays that are accessible from this window that include Moon Phases for the current year, Perigee and Apogee for the current year, and an Almanac listing of the moonrise, moonset and meridian transit times for the current year.
It is possible to toggle between the Sun and Moon windows without returning back to the main window.
Finding the International Space Station (ISS)
Clicking on the "Find ISS" button in the top menu brings up a window showing the future passes of the ISS over the current Observer Location.
Before this computation can proceed, a recent copy of the ISS Two Line Elements (TLE) is required. This can be downloaded from Tools -> Download Earth Satellite TLE in the Main Menu. If this file doesn't exist, or is not within 30 days of the current date, then a warning message is given. Since the downloaded TLE file will only correspond current date of download, then this process cannot be used to locate historical passes, or predict passes far into the future. These TLE files quickly become stale (in-accurate), so a fresh TLE file need to be downloaded on a regular basis.
By clicking on a predicted ISS pass, the the trail of the ISS is displayed on the SkyMap as a pink/brown trail. When a part of the the ISS trail is illuminated by the sun, and the sun is below the horizon, the trail is shown in bold font and illuminated part of the trail is in bold pink.
More details on selecting the Observer Location
Clicking the "Enable Edit UTM" will allow to change the WGS84 UTM values, and allowing for the Lat-Long to be computed from the UTM. To revert back, simply click "Enable Edit Lat-Long".
The map displayed is from the Microsoft Bing Maps API. An internet connection is required to view the map.
The time zone is determined from the selected location, which returns the time zone value for either standard time or summer time (DST) depending on the time of the year. The input information are the latitude and longitude of the selected location. If a location is selected over the ocean, a time zone value computed from a 15 deg segment of longitude is returned.
Up to 10 sets of location data can be saved to the local computer, which will be automatically re-loaded when the app is next restarted. To save a current location setting, click on an empty cell, then select the Save button. If a cell already contains data, it will be over written. To restoring data from a cell, click on the cell to be restored, then select the Restore button.
Keys for the Bing Maps API
The map displayed is from the Microsoft Bing Maps API. This API requires an active internet connection, so if no internet connection is available then the map does not display or update. Additionally, an API key is required for it to function and a temporary key is embedded in the software.
It is possible that at some time in the future, the Bing Maps API key that is embedded in the software will cease to work, either due to expiration or over-use. You will see a message on the map saying "Invalid Credentials. Sign up for a developer account".
In that situation, you can obtain your own key from https://msdn.microsoft.com/en-us/library/ff428642.aspx and load it from the option under the Tools menu. Use your regular Microsoft username and password.
This new personal key may become disconnected from SkyView software when you re-install or update SkyView, so will need to be entered again the SkyView Tools menu.
As far as I know, for fairly limited use, the Bing Maps key is free use up to a certain number of daily transactions (which is quite high), even though it is called a "billable key". Although I dont believe there is any extra cost associated with getting a Bing Maps API key for low level usage, but if there is any extra cost, then this is extra and above any cost that may have been paid to acquire the SkyView app.
When obtaining a key, select Key Type=Basic and Application Type=Public Windows App (8.x and earlier). The Application Name is SkyView.
Date and Time
The default date shown is always the Gregorian date. Even if the date is earlier than 1582, the Gregorian date is still the default date, but the Julian date will be shown alongside. Therefore, if correlating to dates in the Julian era from 45 BCE to 1582, then you need to aware that you have two dates to choose from.
Be aware that other software may automatically flip to the Julian Calendar for dates earlier than 4-Oct-1582, so if comparing results and you see a sudden date difference before 1582, then that is the reason. The Julian and Gregorian calendars become aligned at around the year 250 CE, then drift apart again.
Note that the BCE/CE or BC/AD representation of dates does not include the year zero. Therefore, the BCE representation of a date is one year different from the negative year number.
The Julian Day Number 0 occurs on 01/01/-4712 (or 01-Jan-4713 BCE) in the Julian Calendar, which is also 24/11/-4713 (or 24-Nov-4714 BCE) in the Gregorian calendar.
Time Zones have only existed in their current form for about the last 100 years. Prior to that they were something approximately similar to what they are today. Additionally, some countries have daylight savings and some don’t. The date for changing to daylight savings varies, and some countries have had daylight a one point and now don’t, or vice versa. This all leads to problems when trying to correlate historical dates.
Some examples of complexities of historical time zones follow.
Thailand is a simple example. Currently it is at +7:00 UTC. Prior to 1920 it was at 6:42:04 UTC
Western Australia is slightly more complex. Currently it is as +8:00 UTC and does not have daylight savings time. It had trial periods of daylight savings from 1974-75, 1983-84, 1991-92 and 2006-09. Then prior to 1895 it was at 07:43:24 UTC
England is currently at 0:00 for winter time , 01:00 for summertime. But prior to 1847 was at -0:01:15
Within the SkyView, there are two possibilities for Time Zones that can be selected in the parameters form.
- Assume all times in the past and present have the same time-zone definition as of the current time (today). This is probably the least confusing option.
- Use the correct historical time-zones. This will give the correct historical times for a particular year at that location, but the time values shown may appear erratic.
As an exercise, you can select one of the locations listed above, and notice how the time and time-zones vary as you select some historical dates, and or winter and summer months.
Computations for the Hebrew and Islamic calendars have been introduced. Both these calendars are “lunar calendars”, meaning they follow the moons cycle. They both should be showing the beginning of the month just after a new moon. This is generally the case, but there may be some slight drift after 1000 years or more. These can be allowed or disallowed from the parameters form, as required.
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).
- Atmospheric 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).
- Atmospheric 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”.
Obliquity, Nutation, Precession and Stellar Corrections Plots.
Select either View->Stellar Corrections or View->Obliquity-Nutation to show these plots.
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.
- The Sun and Planets. Good. Using VSOP87C.
- Stars. When the Proper Motion, Aberration, Precession, Nutation and Parallax corrections are applied, the accuracy of the Right Ascension and Declination should be much better than 1 second of an arc.
- Rise, Set and Meridian Transit Times. The meridian transit time should be accurate to within several seconds of time. The Rise and Set times are much less accurate and this is due to the unpredictability of atmospheric refraction near the horizon. As such they should be accurate to within about a minute when at low to moderate values of observer latitude. Accuracy decreases at very high observer latitudes where the sun and moon never rise very high and rise/set at a very low angles relative to the horizon. At these high latitude observer locations, objects spend a longer period of time in the disturbed atmosphere near the horizon while rising/setting, hence the lower accuracy.
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.
An Observer on another Planet
Since the VSOP87 algorithm computes the position of all the planets (Earth included) relative to the barycenter of the Solar System, it's relatively easy to shown the sky as it would be observed from another planet and with similar accuracy to Earth based observation. Some inaccuracy may creep in with values of the obliquity of the ecliptic and longitude of the vernal equinox for the individual planets, since these are not known with great accuracy or how they change with time.
Click on the Observer menu in the main menu, to see these options. Mars has a time reference system defined for it, and this is implemented. Other planets are less well developed (some don't even have a solid surface and the rotational period is not accurately known), so less options are available.
Data from GPS, Glonass, Stations (ISS and Tian Gong), and Iridium satellites are plotted and listed. Other types of satellites may be added in future releases.
- A current version the satellite TLE files (Two Line Element) is required first, and this is done from the Tools -> Download Earth Satellite TLE's.
- The TLE data in these files will go stale after several weeks (or days), so it is good practice to download a refreshed version of these files at regular intervals. When SkyView is started, it gives a summary of the age of the TLE data that is currently in use.
- Select the checkbox for the satellite types that you wish to show. GPS satellite will be shown in blue. Glonass satellites are in red. Stations are Aqua. Iridium are green.
- Trails before and after the satellite can be shown by checking the "Show Trails" box, and selecting the time duration. Note selecting trails will increase the computation load.
- A tabular list is also shown. The colored background depict when a satellite is above the observer’s horizon while the others are below the observers horizon (the Elevation angle is negative when below the horizon). Azimuth and Elevation are in degrees. The Range column is the distance in kilometres from the observer to a satellite.
- If you click on a Satellite in the listing, a black circle will be shown around it on the Satellite plot.
- Since the ISS is in a Low Earth Orbit, It will only be above the horizon for about 5 minutes, several times a day from any one location on the earth’s surface, so it is difficult to catch. Although it can pass overhead at any time of the day, it will only be visible either just before sunrise or just after sunset. If the "Find ISS" task is active, then the time of this plot will be same as the ISS pass.
- 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
- Time keeping on Mars. https://en.m.wikipedia.org/wiki/Timekeeping_on_Mars?wprov=sfla1
Version 2.5.10 22-Feb-2019. Current version as of updating this document.
Version 2.4.7 5-Dec-2018.
Version 2.3.17 5-Oct-18.
Version 2.2.16 31-Jan-2018.
Version 2.1.16 14-Oct-2017.
Version 2.0.40 28-May-2017. First version released to the Microsoft Store.
Version 1.0.0 6-Apr-2016. First draft of SkyView by merging together some earlier software packages such as LatLong-UTM and GPS-Locate.