The next thing you may want to do is change the time and/or date. When KStars starts up, the time is set to your computer's system clock, and the KStars clock is running to keep up with the real time. If you want to stop the clock, select Stop Clock from the Time menu, or simply press the Pause icon in the toolbar. You can make the clock run slower or faster than normal using the time-step spinbox in the toolbar.

You can change to any time or date by selecting Set Time... from the Time menu, or by pressing the hourglass icon in the toolbar. The Set Time window uses a standard KDE Date Picker widget, coupled with three spinboxes for setting the hours, minutes and seconds. If you ever need to reset the clock back to the current time, just select Set Time to Now from the Time menu.

Now that we have the time and location set, let's have a look around. You can pan the display using the arrow keys. If you hold down the Shift key before panning, the scrolling speed is doubled. The display can also be panned by clicking and dragging with the mouse. Note that while the display is scrolling, not all objects are displayed. This is done to cut down on the CPU load of recomputing object positions, which makes the scrolling smoother. You can zoom in and out with the + and - keys, with the zoom in/out buttons in the toolbar, or by selecting Zoom In/Zoom Out in the View menu. Notice that as you zoom in, you can see fainter stars than at lower zoom settings.

Zoom out until you can see a green curve; this represents your local horizon. If you haven't adjusted the KStars configuration, the display will be solid green below the horizon, representing the solid ground of the Earth. There is also a white curve, which represents the celestial equator (an imaginary line which divides the sky into northern and southern hemispheres). There is also a tan curve, which represents the Ecliptic, the path that the Sun appears to follow across the sky over the course of a year. Therefore, the Sun is always found somewhere along the Ecliptic, and the planets are never far from it.

KStars displays thousands of objects: stars, planets, clusters, nebulae and galaxies. You can identify any object by clicking on it (its name appears in the statusbar at the bottom of the window). You can get more information about an object by right-clicking on it. A popup menu will appear, showing its name(s), object type, and links to images and information from the internet.

If you know of an additional URL with information or an image of the object, you can add a custom link to the object's popup menu using the Add Link... item. This opens a window in which you can enter the URL and the text that should appear in the popup menu. You can make sure the URL is correct with the Check URL button, which will test the URL in your web browser. Please specify whether the URL points to an Image, or to an HTML document! If you specify Image here, the new menu item will open the Image Viewer, not the web browser. You can also point to files on your local disk, so this feature could be used to attach observing logs or other custom information to objects in KStars. Your custom links are automatically loaded whenever KStars starts up, and they are stored in the directory ~/.kde/share/apps/kstars/, in files myimage_url.dat and myinfo_url.dat.

You can search for named objects by clicking on the search icon in the toolbar, or by selecting Find Object... from the Location menu. The Find Object window lists all the named objects in the KStars database. Many objects are listed only by their catalog name (for example, NGC 3077), but some are also listed by their common name (for example, Whirlpool Galaxy). You can filter the list by name, or by object type. Highlight the desired object, and press Ok. The display will center on the object and begin tracking it. Note that if the object is below the horizon, you may not see anything except the ground. You can make the ground invisible in the Display Options window.

Object Tracking is automatically engaged whenever an object is centered in the display, either by using the Find Object window, by double-clicking on an object, or by selecting Center and Track from the right-click popup menu. You can disengage tracking by panning the display, pressing the Lock icon in the toolbar, or selecting Track Object from the Location menu.

The current RA, Dec coordinates of the mouse cursor is always displayed in the status bar. Clicking the Mouse anywhere will identify the nearest object in the status bar. Double-clicking will center and track on the clicked location or object. Click and drag to pan the display. Right click to bring up the popup menu with detailed options for the clicked object.

A basic requirement for studying the heavens is determining where in the sky things are. To specify sky positions, astronomers have developed several coordinate systems. Each uses a coordinate grid projected on the Celestial Sphere, in analogy to the Geographic coordinate system used on the surface of the Earth. The coordinate systems differ only in their choice of the fundamental plane, which divides the sky into two equal hemispheres along a great circle. (the fundamental plane of the geographic system is the Earth's equator). Each coordinate system is named for its choice of fundamental plane.

The sky appears to drift overhead from east to west, completing a full circuit around the sky in 24 (Sidereal) hours. This phenomenon is due to the spinning of the Earth on its axis. The Earth's spin axis intersects the Celestial Sphere at two points. These points are the Celestial Poles. As the Earth spins; they remain fixed in the sky, and all other points seem to rotate around them. The celestial poles are also the poles of the Equatorial Coordinate System, meaning they have Declinations of +90 degrees and -90 degrees (for the North and South celestial poles, respectively).

The North Celestial Pole currently has nearly the same coordinates as the bright star Polaris (which is Latin for "Pole Star"). This makes Polaris useful for navigation: not only is it always above the North point of the horizon, but its Altitude angle is always (nearly) equal to the observer's Geographic Latitude (however, Polaris can only be seen from locations in the Northern hemisphere).

The fact that Polaris is near the pole is purely a coincidence. In fact, because of Precession, Polaris is only near the pole for a small fraction of the time.

The celestial sphere is an imaginary sphere of gigantic radius, centered on the Earth. All objects which can be seen in the sky can be thought of as lying on the surface of this sphere.

Of course, we know that the objects in the sky are not on the surface of a sphere centered on the Earth, so why bother with such a construct? Everything we see in the sky is so very far away, that their distances are impossible to gauge just by looking at them. Since their distances are indeterminate, you only need to know the direction toward the object to locate it in the sky. In this sense, the celestial sphere model is a very practical model for mapping the sky.

The directions toward various objects in the sky can be quantified by constructing a Celestial Coordinate System.

The ecliptic is an imaginary Great Circle on the Celestial Sphere along which the Sun appears to move over the course of a year. Of course, it is really the Earth's orbit around the Sun causing the change in the Sun's apparent direction. The ecliptic is inclined from the Celestial Equator by 23.5 degrees. The two points where the ecliptic crosses the celestial equator are known as the Equinoxes.

Since our solar system is relatively flat, the orbits of the planets are also close to the plane of the ecliptic. In addition, the constellations of the zodiac are located along the ecliptic. This makes the ecliptic a very useful line of reference to anyone attempting to locate the planets or the constellations of the zodiac, since they all literally “follow the Sun”.

The Altitude of the ecliptic above the Horizon changes over the course of the year, because of the 23.5 degree tilt of the Earth's spin axis. This causes the seasons. When the ecliptic (and therefore the Sun) is high above the horizon, the days are longer, and you have Summer. When the ecliptic is low in the sky, you have Winter.

Most people know the Vernal and Autumnal Equinoxes as calendar dates, signifying the beginning of the Northern hemisphere's Spring and Autumn, respectively. Did you know that the equinoxes are also positions in the sky?

The Celestial Equator and the Ecliptic are two Great Circles on the Celestial Sphere, set at an angle of 23.5 degrees. The two points where they intersect are called the Equinoxes. The Vernal Equinox has coordinates RA=0.0 hours, Dec=0.0 degrees. The Autumnal Equinox has coordinates RA=12.0 hours, Dec=0.0 degrees.

The Equinoxes are important for marking the seasons. Because they are on the Ecliptic, the Sun passes through each equinox every year. When the Sun passes through the Vernal Equinox (usually on March 21st), it crosses the Celestial Equator from South to North, signifying the end of Winter for the Northern hemisphere. Similarly, when the Sun passes through the Autumnal Equinox (usually on September 21st), it crosses the Celestial Equator from North to South, signifying the end of Winter for the Southern hemisphere.

Locations on Earth can be specified using a spherical coordinate system. The geographic (“earth-mapping”) coordinate system is aligned with the spin axis of the Earth. It defines two angles measured from the center of the Earth. One angle, called the Latitude, measures the angle between any point and the Equator. The other angle, called the Longitude, measures the angle along the Equator from an arbitrary point on the Earth (Greenwich, England is the accepted zero-longitude point in most modern societies).

By combining these two angles, any location on Earth can be specified. For example, Baltimore, Maryland (USA) has a latitude of 39.3 degrees North, and a longitude of 76.6 degrees West. So, a vector drawn from the center of the Earth to a point 39.3 degrees above the Equator and 76.6 degrees west of Greenwich, England will pass through Baltimore.

The Equator is obviously an important part of this coordinate system, it represents the zeropoint of the latitude angle, and the halfway point between the poles. The Equator is the Fundamental Plane of the geographic coordinate system. All Spherical Coordinate Systems define such a Fundamental Plane.

Lines of constant Latitude are called Parallels. They trace circles on the surface of the Earth, but the only parallel that is a Great Circle is the Equator (Latitude=0 degrees). Lines of constant Longitude are called Meridians. The Meridian passing through Greenwich is the Prime Meridian (longitude=0 degrees). Unlike Parallels, all Meridians are great cricles, and Meridians are not parallel: they intersect at the north and south poles.

Consider a sphere, such as the Earth, or the Celestial Sphere. The intersection of any plane with the sphere will result in a circle on the surface of the sphere. If the plane happens to contain the center of the sphere, the intersection circle is a Great Circle. Great circles are the largest circles that can be drawn on a sphere. Also, the shortest path between any two points on a sphere is always along a great circle.

Some examples of great circles on the celestial sphere include: the Horizon, the Celestial Equator, and the Ecliptic.

The Horizon is the line that separates Earth from Sky. More precisely, it is the line that divides all of the directions you can possibly look into two categories: those which intersect the Earth, and those which do not. At many locations, the Horizon is obscured by trees, buildings, mountains, etc. However, if you are on a ship at sea, the Horizon is strikingly apparent.

The horizon is the Fundamental Plane of the Horizontal Coordinate System. In other words, it is the locus of points which have an Altitude of zero degrees.

As explained in the Sidereal Time article, the Right Ascension of an object indicates the Sidereal Time at which it will transit across your Local Meridian. An object's Hour Angle is defined as the difference between the current Local Sidereal Time and the Right Ascension of the object:

HA_obj = LST - RA_obj

Thus, the object's Hour Angle indicates how much Sidereal Time has passed since the object was on the Local Meridian. It is also the angular distance between the object and the meridian, measured in hours (1 hour = 15 degrees). For example, if an object has an hour angle of 2.5 hours, it transited across the Local Meridian 2.5 hours ago, and is currently 37.5 degrees West of the Meridian. Negative Hour Angles indicate the time until the next transit across the Local Meridian. Of course, an Hour Angle of zero means the object is currently on the Local Meridian.

The Julian Calendar is a way of reckoning the current date by a simple count of the number of days that have passed since some remote, arbitrary date. This number of days is called the Julian Day, abbreviated as JD. The starting point, JD=0, is January 1, 4713 BC (or -4712 January 1, since there was no year '0'). Julian Days are very useful because they make it easy to determine the number of days between two events by simply subtracting their Julian Day numbers. Such a calculation is difficult for the standard (Gregorian) calendar, because days are grouped into months, which can contain a variable number of days, and there is the added complication of Leap Years.

Converting from the standard (Gregorian) calendar to Julian Days and vice versa is best left to a special program written to do this, and there are many to be found on the web (and KStars does this too, of course!). However, for those interested, here is a simple example of a Gregorian to Julian day converter:

JD = D - 32075 + 1461*( Y + 4800 * ( M - 14 ) / 12 ) / 4 + 367*( M - 2 - ( M - 14 ) / 12 * 12 ) / 12 - 3*( ( Y + 4900 + ( M - 14 ) / 12 ) / 100 ) / 4

where D is the day (1-31), M is the Month (1-12), and Y is the year (1801-2099). Note that this formula only works for dates between 1801 and 2099. More remote dates require a more complicated transformation.

An example Julian Day is: JD 2440588, which corresponds to 1 Jan, 1970.

Julian Days can also be used to tell time; the time of day is expressed as a fraction of a full day, with 12:00 noon (not midnight) as the zero point. So, 3:00 pm on 1 Jan 1970 is JD 2440588.125 (since 3:00 pm is 3 hours since noon, and 3/24 = 0.125 day). Note that the Julian Day is always determined from Universal Time, not Local Time.

Astronomers use certain Julian Day values as important reference points, called Epochs. One widely-used epoch is called J2000; it is the Julian Day for 1 Jan, 2000 at 12:00 noon = JD 2451545.0.

Much more information on Julian Days is availabel on the internet. A good starting point is the U.S. Naval Observatory. If that site is not available when you read this, try searching for “Julian Day” with your favorite search engine.

The Earth has two major components of motion. First, it spins on its rotational axis; a full spin rotation takes one Day to complete. Second, it orbits around the Sun; a full orbital rotation takes one Year to complete.

There are normally 365 days in one calendar year, but it turns out that a true year (i.e., a full orbit of the Earth around the Sun; also called a tropical year) is a little bit longer than 365 days. In other words, in the time it takes the Earth to complete one orbital circuit, it completes 365.24219 spin rotations. Don't be too suprised by this; there's no reason to expect the spin and orbital motions of the Earth to be synchronized in any way. However, it does make marking calendar time a bit awkward!

What would happen if we simply ignored the extra 0.24219 rotation at the end of the year, and simply defined a calendar year to always be 365.0 days long? The calendar is basically a charting of the Earth's progress around the Sun. If we ignore the extra bit at the end of each year, then with every passing year, the calendar date lags a little more behind the true position of Earth around the Sun. In a few centuries, Winter will begin in September!

In fact, it used to be that all years were defined to have 365.0 days, and the calendar “drifted” away from the true seasons as a result. In the year 46 BCE, Julius Caeser established the Julian Calendar, which implemented the world's first leap years: He decreed that every 4th year would be 366 days long, so that a year was 365.25 days long, on average. This basically solved the calendar drift problem.

However, the problem wasn't completely solved by the Julian calendar, because a tropical year isn't 365.25 days long; it's 365.24219 days long! You still have a calendar drift problem, it just takes many centuries to become noticeable. And so, in 1582, Pope Gregory XIII instituted the Gregorian calendar, which was largely the same as the Julian Calendar, with one more trick added for leap years: even Century years (those ending with the digits "00") are only leap years if they are divisible by 400. So, the years 1700, 1800, and 1900 were not leap years (though they would have been under the Julian Calendar), whereas the year 2000 was a leap year. This change makes the average length of a year 365.2425 days. So, there is still a tiny calendar drift, but it amounts to an error of only 3 days in 10,000 years! The Gregorian calendar is still used as a standard calendar throughout most of the world.

The Meridian is an imaginary Great Circle on the Celestial Sphere that is perpendicular to the local Horizon. It passes through the North point on the Horizon, through the Celestial Pole, up to the Zenith, and through the South point on the Horizon.

Because it is fixed to the local Horizon, stars will appear to drift past the Local Meridian as the Earth spins. You can use an object's Right Ascension and the Local Sidereal Time to determine when it will cross your Local Meridian (see Hour Angle).

Precession is the gradual change in the direction of the Earth's spin axis. The spin axis traces a cone, completing a full circuit in 26,000 years. If you've ever spun a top or a dreidel, the “wobbling” rotation of the top as it spins is precession.

Because the direction of the Earth's spin axis changes, so does the location of the Celestial Poles.

The reason for the Earth's precession is complicated. The Earth is not a perfect sphere, it is a bit flattened, meaning the Great Circle of the equator is longer than a “meridonal” great circle that passes through the poles. Also, the Moon and Sun lie outside the Earth's Equatorial plane. As a result, the gravitational pull of the Moon and Sun on the oblate Earth induces a slight torque in addition to a linear force. This torque on the spinning body of the Earth leads to the precessional motion.

Retrograde Motion is the orbital motion of a body in a direction opposite that which is normal to spatial bodies within a given system.

When we observe the sky, we expect most objects to appear to move in a particular direction with the passing of time. The apparent motion of most bodies in the sky is from east to west. However it is possible to observe a body moving west to east, such as an artificial satellite or space shuttle that is orbiting eastward. This orbit is considered Retrograde Motion.

Retrograde Motion is most often used in reference to the motion of the outer planets (Mars, Jupiter, Saturn, and so forth). Though these planets appear to move from east to west on a nightly basis in response to the spin of the Earth, they are actually drifting slowly eastward with respect to the stationary stars, which can be observed by noting the position of these planets for several nights in a row. This motion is normal for these planets, however, and not considered Retrograde Motion. However, since the Earth completes its orbit in a shorter period of time than these outer planets, we occassionally overtake an outer planet, like a faster car on a multiple-lane highway. When this occurs, the planet we are passing will first appear to stop its eastward drift, and it will then appear to drift back toward the west. This is Retrograde Motion, since it is in a direction opposite that which is typical for planets. Finally as the Earth swings past the the planet in its orbit, they appear to resume their normal west-to-east drift on succesive nights.

This Retrograde Motion of the planets puzzled ancient Greek astronomers, and was one reason why they named these bodies “planets” which in Greek means “wanderers”.

Sidereal Time literally means “star time”. The time we are used to using in our everyday lives is Solar Time. The fundamental unit of Solar Time is a Day: the time it takes the Sun to travel 360 degrees around the sky, due to the rotation of the Earth. Smaller units of Solar Time are just divisions of a Day:

However, there is a problem with Solar Time. The Earth doesn't actually spin around 360 degrees in one Solar Day. The Earth is in orbit around the Sun, and over the course of one day, it moves about one Degree along its orbit (360 degrees/365.25 Days for a full orbit = about one Degree per Day). So, in 24 hours, the direction toward the Sun changes by about a Degree. Therefore, the Earth has to spin 361 degrees to make the Sun look like it has traveled 360 degrees around the Sky.

In astronomy, we are concerned with how long it takes the Earth to spin with respect to the “fixed” stars, not the Sun. So, we would like a timescale that removes the complication of Earth's orbit around the Sun, and just focuses on how long it takes the Earth to spin 360 degrees with respect to the stars. This rotational period is called a Sidereal Day. On average, it is 4 minutes shorter than a Solar Day, because of the extra 1 degree the Earth spins in a Solar Day. Rather than defining a Sidereal Day to be 23 hours, 56 minutes, we define Sidereal Hours, Minutes and Seconds that are the same fraction of a Day as their Solar counterparts. Therefore, one Solar Second = 1.00278 Sidereal Seconds.

The Sidereal Time is useful for determining where the stars are at any given time. Sidereal Time divides one full spin of the Earth into 24 Sidereal Hours; similarly, the map of the sky is divided into 24 Hours of Right Ascension. This is no coincidence; Local Sidereal Time (LST) indicates the Right Ascension on the sky that is currently crossing the Local Meridian. So, if a star has a Right Ascension of 05h 32m 24s, it will be on your meridian at LST=05:32:24. More generally, the difference between an object's RA and the Local Sidereal Time tells you how far from the Meridian the object is. For example, the same object at LST=06:32:24 (one Sidereal Hour later), will be one Hour of Right Ascension west of your meridian, which is 15 degrees. This angular distance from the meridian is called the object's Hour Angle.

The Earth is round, and it is always half-illuminated by the Sun. However, because the Earth is spinning, the half that is illuminated is always changing. We experience this as the passing of days wherever we are on the Earth's surface. At any given instant, there are places on the Earth passing from the dark half into the illuminated half (which is seen as dawn on the surface). At the same instant, on the opposite side of the Earth, points are passing from the illuminated half into darkness (which is seen as dusk at those locations). So, at any given time, different places on Earth are experiencing different parts of the day. Thus, Solar time is defined locally, so that the clock time at any location describes the part of the day consistently.

This localization of time is accomplished by dividing the globe into 24 vertical slices called Time Zones. The Local Time is the same within any given zone, but the time in each zone is one Hour earlier than the time in the neighboring Zone to the East. Actually, this is a idealized simplification; real Time Zone boundaries are not straight vertical lines, because they often follow national boundaries and other political considerations.

Note that because the Local Time always increases by an hour when moving between Zones to the East, by the time you move through all 24 Time Zones, you are a full day ahead of where you started! We deal with this paradox by defining the International Date Line, which is a Time Zone boundary in the Pacific Ocean, between Asia and North America. Points just to the East of this line are 24 hours behind the points just to the West of the line. This leads to some interesting phenomena. A direct flight from Australia to California arrives before it departs! Also, the islands of Fiji straddle the International Date Line, so if you have a bad day on the West side of Fiji, you can go over to the East side of Fiji and have a chance to live the same day all over again!

The time on our clocks is essentially a measurement of the current position of the Sun in the sky, which is different for places at different Longitudes because the Earth is round (see Time Zones).

However, it is sometimes necessary to define a global time, one that is the same for all places on Earth. One way to do this is to pick a place on the Earth, and adopt the Local Time at that place as the Universal Time, abbrteviated UT. (The name is a bit of a misnomer, since Universal Time has little to do with the Universe. It would perhaps be better to think of it as global time).

The geographic location chosen to represent Universal Time is Greenwich, England. The choice is arbitrary and historical. Universal Time became an important concept when European ships began to sail the wide open seas, far from any known landmarks. A navigator could reckon the ship's longitude by comparing the Local Time (as measured from the Sun's position) to the time back at the home port (as kept by an accurate clock on board the ship). Greenwich was home to England's Royal Observatory, which was charged with keeping time very accurately, so that ships in port could re-calibrate their clocks before setting sail.

The Zenith is the point in the sky where you are looking when you look “straight up” from the ground. More precisely, it is the point on the sky with an Altitude of +90 Degrees; it is the Pole of the Horizontal Coordinate System. Geometrically, it is the point on the Celestial Sphere intersected by a line drawn from the center of the Earth through your location on the Earth's surface.

The Zenith is, by definition, a point along the Local Meridian.