The shape of a planet's orbit and the planet's position in that orbit is defined by the planet's orbital elements. If you know the orbital elements for a planet, moon, comet, asteroid or satellite then you have enough information to add that object to Starry Night. To enter the orbital elements for a new object, open the orbit editor.
Style: Orbital elements come in a number of different formats depending on who is supplying the elements are for what type of object they are being supplied for. You can enter the orbital elements for any object using any format you want. Starry Night lets you switch between the various formats for the orbital elements with the Style popup. Since planets, moons and asteroids are most often in near-circular orbits their orbital elements are normally given in a way that best represents these types of orbits. Comets on the other hand often have highly eccentric orbits and their elements are normally given in what Starry Night calls Perircentric style. NASA's standard way of describing a satellite's orbit is using a concatenated form called a NASA two line element (or TLE for short).
Ref Plane: In order to specify orbits, or for that matter, the position of any astronomical object, it is necessary to have a reference system. There are two standard reference planes, the equatorial plane of the Earth and the ecliptic plane (the plane of Earth's orbit around the Sun). However, due to precession, the equatorial and the ecliptic planes are slowly changing their positions relative to the background stars. Consequently, an astronomical reference plane is dependent upon the time of the observations. The Ref Plane popup menu allows you to select between standard ecliptic and equatorial references planes.
- Ecliptic: An ecliptic reference plane based on the time of your current Starry Night window.
- Ecliptic 1950: An ecliptic reference plane based on the J1950.0 standards.
- Ecliptic 2000: An ecliptic reference plane based on the J2000.0 standards, established by the International Astronomical Union (IAU) in 1976.
- Earth Equatorial: Reference plane based on the time of your current Starry Night window using Earth's equatorial coordinate system.
- Earth Equatorial 1950: Reference plane based on the J1950.0 standards using Earth's equatorial coordinate system.
- Earth Equatorial 2000: Reference plane based on the J2000.0 coordinates, established by the IAU in 1976 using Earth's equatorial coordinate system.
- Planet Equatorial: Reference plane based on the time of your current Starry Night window using the equatorial coordinate system of whatever body this object is orbiting.
Pericentric and Near-Circular Orbital Elements
Adjusting sliders and entering orbital elements into data boxes is relatively easy. Understanding what these numbers represent is a little more difficult. Orbital elements remain a mystery to most people, due in part to the complex names these numbers have acquired, and secondly to the trouble many people have in thinking three-dimensionally. To make matters even more complicated, often an orbital element will have several different names.
Several numbers are required to establish an object's orbit. These orbital elements, first defined by Johannes Kepler at the turn of the 17th century, place an object on an elliptical path at a particular time, and orient it about a parent body.
- Mean Distance: Kepler's third law of orbital motion gives us a precise relationship between the speed of a satellite and its distance from the parent. Objects that are close to the parent orbit quickly, while objects farther away orbit more slowly. The implication is that if we specify either the speed at which the object is moving or its distance from the parent, we've measured similar values. In effect, Mean Distance and Mean Motion are two ways of describing the same thing.
The convention with planets is to call this number the Mean Distance. Planets in circular orbits would travel at a constant distance from their parent body, but since most planetary orbits are elliptical, this distance is constantly changing. The common practice is to average this distance, and record it as Mean Distance. It is usually measured in AUs.
The convention with satellites is to call this number the Mean Motion. Satellites in circular orbits travel at a constant speed, but since most orbits are elliptical, their speed is constantly changing as they orbit. The common practice is to average the speed, call it the Mean Motion, and record it in units of revolutions per day.
Comets orbits are extremely elliptical, so the distance between comet and parent body is usually measured at pericenter, the point in their orbit where they are closest to the parent. This distance is called the Pericenter distance, and is given in units of AU.
For objects in orbit about the Sun, such as comets, pericenter distance is called Perihelion distance.
- Eccentricity: Eccentricity describes the shape of the orbit, based on a ratio of the distance of the focus from the center of the orbit's ellipse to the length of its semi-major axis. A circular orbit has an eccentricity of 0, while an extremely elliptical orbit, such as a that of a comet, has a value close to 1. The eccentricity of hyperbolic orbits is greater than 1.
- Inclination: The orbit's elliptical shape lies in a plane known as the orbital plane. The orbital plane always goes through the center of the parent object, but may be tilted at any angle relative to the parent's equator. Inclination is the angle between the orbital plane and the equatorial plane, measured between 0° and 180°. If the orbit lies in the ecliptic plane, the inclination is 0°. At 90°, the orbit is perpendicular to the ecliptic, while an inclination of greater than 90° describes a retrograde orbit.
- Satellites with an inclination near 0° are said to have equatorial orbits, because the satellite orbits around the equator. Those whose orbits are inclined near 90° are called polar, because the satellite crosses over the north and south poles.
- Ascending Node: This angular measurement specifies the point at which the orbit crosses northward through the ecliptic plane.
- The ascending node is also sometimes called the Longitude of the Ascending Node, and is measured from the Prime Meridian of the parent body.
- Argument of Pericenter: The pericenter is the point on the orbit which is closest to the parent body. The Argument of Pericenter specifies the angular location of the pericenter, and is measured in degrees.
- The value is determined by measuring the angle (measured at the center of the parent) from the ascending node to pericenter. For example, when the Argument of Pericenter is 0°, the pericenter occurs at the same place as the ascending node. That means that the planet would be closest to the Sun just as it rises up through the ecliptic plane. Likewise, when the Argument of Pericenter is 180°, the planet, as it rises up through the ecliptic plane, is at its farthest from Sun.
- Mean Anomaly: Mean Anomaly describes exactly where on the orbit the new object is located at the specified time. It is measured as an angle over one revolution, starting from 0° at the pericenter.
- Epoch: A set of orbital elements is a portrait of an orbit, at a specific time. The Epoch specifies this time. In most cases, this time is expressed as a Julian date. However, NASA has its own epoch system that is commonly used for describing satellite orbits. Its format lists the year, the number of days, then the percentage of the day. For example 1997045.5 would translate as February 14th, 1997, at 12 hours UT.
Orbit Editor Calculations:
You may notice that some numbers you enter may change when switching between different styles. For instance, if you've entered 485° in an Ascending Node box, move to another style, then return, the number will have changed to 125°. Starry Night has recalculated the number, but in effect, the value of the orbital element remains the same. The new number displayed is mathematically equivalent to the original number that you entered.
You also may notice that sliders may change when adjusting certain elements. This is because Starry Night is recalculating the position of the sliders.
For instance, if you adjust the Rotation rate of an object using the sliders, the Meridian slider will jump to a new position. Note that the Meridian drawn on the object has not moved. Starry Night has recalculated the Meridian position to keep it synchronized with J2000 standards.
Note: Data box entries are not recalculated in such a fashion.
Converting Orbit Editor Dates:the Orbit Editor requires you to input dates as a Julian day value. If the original date is in ordinary format, there is an easy way to convert it to a Julian day.
- Enter the ordinary date into the Time palette.
- Click the Set Julian button.
- Copy the Julian date and close the dialog.
- Go back to the Orbit Editor and paste in the Julian date.
- To find the NASA epoch, switch to the AMSAT style. Starry Night automatically converts the date for you.
You can also perform similar actions in order to convert a Julian date or NASA date to a normal date.
- Plug your NASA epoch into the appropriate AMSAT data box,
- Switch styles to Pericentric.
- Copy the Julian day from the Epoch data box.
- Press the Julian button on the Time Palette.
- Paste in the new Julian day.
Set the time. The Time palette displays the Julian date as a normal time.
Brightest Satellites: This page tracks the 100 brightest satellites and gives their NASA two line elements (TLE).
Iridium Satellites: Iridium bills itself as the worlds first handheld glabal satellite telephone and paging network. The satellites that were put into orbit for this network can be tracked using the TLEs found on this page. More Iridium info.
Other Satellites: A good collection of weather, navigation, military, and communications satellite elements from the Celestrack website.