The Orbit Editor

 

The Orbit Editor is one of the most exciting features of Starry Night Deluxe. Using it, you can add and edit new objects in your virtual Solar System. For instance, if a comet or moon has been recently discovered, you can use the Orbit Editor to add it to Starry Night by simply entering its orbital elements. The Orbit Editor also allows you to add surface and atmosphere maps to your new objects.

Adding your own objects to the Solar system is a great way to learn about celestial mechanics. The Orbit Editor lets you view graphically the shapes, sizes, and positions of a new orbit, as well as adjust these aspects in realtime with sliders. This feature makes it clear what each particular orbital element means, and how the orbit is affected when adjustments are made.

This chapter is divided into two sections. The first will give you a quick guide to the editor so that you can get on with the job of adding new planets. The second section deals with the editor in greater detail -- explaining all the technical options.

  • Note: The Orbit Editor Window defaults to using an Ecliptic Orientation. As a result, horizons are not shown. For more information, please see the description of Orientation in the chapter Setting Menu.

 

Section 1

The Orbit Editor Window

To use the Orbit Editor, open the Planet Palette and, using the Selection tool, select the body (the Sun, a planet, or a moon) that you want to place a new object around. Then press the Add button to open the Orbit Editor Window.

The window opens with a default view of your new orbit, as well as a series of tabs, each containing orbit options and adjustable settings.

Starry Night calculates a preferred field of view for each tab folder, one that reflects the function of the tab. For example, for practical reasons, the view of your new object from the Surface tab will be closer than that of the view from the Orbital Elements tab.

  • Tip: The Orbit Editor window is a fully functional Starry Night window. You can use all of Starry Night's commands and tools to adjust your view of it.


Info Tab

From within this tab, you can name your object, and from a popup menu, choose the kind of object it will be. You can also adjust the orbit color, and the Absolute Magnitude of the object. (Absolute Magnitude is the brightness of the object seen from 1 AU, with the Sun at a distance of 1 AU.)


Orbital Elements Tab

This second tab folder contains popup menus, sliders and data input boxes which control the attributes of the new orbit.

Style

This popup menu lets you choose the style of orbital data that you'll be entering. It automatically defaults to the style most common to the type of object you have selected in the Info Tab folder. If you wish to change this, use this popup menu.

Depending on the style chosen, you are presented with sliders and/or data entry boxes into which you can plug in your new object's orbital elements.

  • Note: Comet tail lengths are determined as a result of both absolute magnitude and size of the comet nucleus (planet size).

Reference 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 popup menu allows you to select from ecliptic and equatorial standards from which all of the orbital element data are measured.

Ecliptic -- an ecliptic reference plane based on the time of your current Starry Night window.

Ecliptic 1950 -- an ecliptic reference plane based on the B1950.0 standards.

Ecliptic 2000 -- an ecliptic reference plane based on the J2000.0 standards, established by the International Astronomical Union (IAU) in 1976.

Equatorial -- an equatorial reference plane based on the time of your current Starry Night window.

Equatorial 1950 -- an ecliptic reference plane based on the B1950.0 standards.

Equatorial 2000 -- an equatorial reference plane based on the J2000.0 coordinates, established by the IAU in 1976.


Axis/Rotation/Size Tab

Using these sliders and data boxes, you can enter information about your new planet's rotation rate, pole positions, and diameter.

Rotation -- These settings allow you to establish how your new object will rotate.

Meridian Position -- sets the position of the meridian line on the object, in J2000 equatorial coordinates.

Rotation Rate -- adjusts the rate of rotation that your new object will have.

Set Spin to Face Parent -- Over time, many objects in the solar system become gravitationally locked, that is, they keep the same face towards their parent at all times. The Moon and Io are good examples of this. Clicking this button will automatically "lock" your object, that is, set its rotation to match its orbital period.


Surface Tab

Within this tab folder, you can customize your new planet's color, surface, and by adjusting atmosphere settings.

Set Color

By default, the color of your new object is the same as the parent object. If you wish to adjust this color, press the Set Color button.

Surface maps

The two rectangular windows display the surface and atmosphere maps of your new planet. They will be blank until you copy and paste an image into them.

Adding images to your new objects can be a lot of fun. Besides using the latest photos from space probes, you can add any picture you like: your pet's, your family, a corporate logo, you name it! Just copy any image to your clipboard, then use the orbit editor to paste it onto your new object.

  • Note: Starry Night's Prime Directive states that the major moons and planets that ship with Starry Night cannot be edited using the Orbit Editor.

No Atmosphere

This window displays the map used whenever "Show Atmosphere" is not checked in Options>Planets>Surface Drawing under the Settings menu.

With Atmosphere

This window displays the map used whenever "Show Atmosphere" is checked in Options>Planets>Surface Drawing under the Settings menu.

Copy and Pasting images

By using the Copy, Paste, and Clear buttons, you can add and edit maps.

Copy -- This copies the contents of the map window to your computer's clipboard. You can use this feature to copy the maps of other planets if you wish to add them to your new object. To do this select a planet on the Planet Palette and push the Open button that appears at the bottom of the palette. Then go to the surface made and press the copy button.

Paste -- This pastes the contents of the computer's clipboard into the map window.

Clear -- This clears the map window, without copying to the clipboard.

The Orbit Editor can use almost any image as a planet map, as long as you can copy it to the clipboard. It can even use the images selected and copied with the picture selection tool.

Image size and appearance

There is some restriction on size, the largest the image can be is 1500 by 1500 pixels, the smallest it can be is 25 by 25 pixels (if the picture is too big or too small, your computer will beep twice and not allow the paste). We recommend using images that are about 600 pixels long by 300 pixels high.

Whatever picture you do use, the width of it will be wrapped around the planet, and the height of it will go from pole to pole. Because it's being mapped on a sphere, your image may be distorted in the northern and southern latitudes. Keep the important parts of your image near the "equator". Take a look at the images of the planets or the comets (in the Orbit Editor or using the Location window) to get an idea of how an object's map relates to its appearance as a wrapped planet.

Seams on objects

If you have an image-editing program, such as Adobe Photoshop, you can adjust your images so that the seam (where the end of the pictures meet when wrapped on a planet) is minimized or invisible. Select one end of the picture (for example, the right third of the image), and Cut it. Then slide the remaining two-thirds of the image all the way over to the right. Paste in the first piece, and position it so that it is now the left side of the image. You should now have an obvious seam where the two pieces are joined. At this point, you can use the image-editing program to clean up the seam, blending the two sides together so that the join is less obvious. Now select the entire picture and copy it. Then, using the Orbit Editor, paste it into your Starry Night object. The place on your planet where the two sides of the image meet should now be seamless.


Example: Adding a new planet between the orbits of Mars and Jupiter

  1. Open a new window.
  2. Open the Planet Palette, then select the Sun.
  3. Press the Add button at the bottom of the Planet Palette.
  4. The Orbit Editor Window will open, giving you the default view of your new object's orbit. Since you've selected the Sun, the default orbit is just past Earth's.
  5. In the Info Tab, select "Planet" from the Type of Object popup menu.
  6. Give your new planet a name.
  7. Adjust the orbit color and Absolute Magnitude if desired.
  8. Open the Orbital Elements tab
  9. Using the Planet Palette, turn on Mars' orbit. (By default, Jupiter's orbit is already on.)
  10. Use the elevation up button on the Tool palette to rocket out just past Jupiter's orbit (about 10 AU's). Here you'll have a better view by which to adjust your new planet's orbit.
  11. Using the Mean Distance slider, adjust your planet's orbit so that it lies in between Mars and Jupiter's.

 

Section 2

Understanding 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.


Orbital Elements

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 orientate it about a parent body.

  • Note: The real world is slightly more complex than the Keplerian model, since there are other factors that can influence an orbit, including the gravitation influence of other planets, gravity anomalies of the parent, and atmospheric drag on an object if it is in a low orbit.

Mean Distance / Mean Motion / Pericenter Distance: How far away is the object from the parent?

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 parents, 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 given in units of AU.

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.

Pericenter distance is also often called Perihelion distance (for objects in orbit about the Sun).

Inclination: The tilt of the orbit

The orbit's ellipse 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 called equatorial orbits (because the satellite stays nearly over the equator). Those whose orbits are inclined near 90 are called polar (because the satellite crosses over the north and south poles).

  • Nerd stuff: Spy satellites are often polar orbiting. In this inclination, they can examine all parts of the Earth as it rotates underneath them.

Ascending Node / Right Ascension of Ascending Node: Where the orbit cross the ecliptic equator

This measurement specifies the point at which the orbit crosses the ecliptic plane on the ascending node (where the object crosses up through the ecliptic plane going from south to north).

For satellites, the convention is to specify the Right Ascension of the Ascending Node. The RAAN of a satellite's orbit is the angle (measured at the center of the Earth) between the vernal equinox and the place where the satellite's orbit rises up through the equator.

The ascending node is also sometimes called the Longitude of the Ascending Node.

Argument of Pericenter/Perigee: The orbit's closest point

The pericenter of a planet is the point on its orbit that it is closest to the Sun. The Argument of Pericenter describes where on the orbit the pericenter will be, and is measured in degrees.

The point where a satellite is closest to the parent is called the perigee, so, for satellites, the convention is to specify the Argument of Perigee.

The value is determined by measuring the angle (measured at the center of the parent) from the ascending node to pericenter/perigee. 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, would be at its farthest from Sun.

Eccentricity: The orbit's shape

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 would have an eccentricity of 0, while an extremely elliptical orbit (such as a comet's) would have a value close to 1.

Mean Anomaly: Where the new object is located

Mean anomaly is used to describe exactly where on the orbit the new object is located, at the specified time. It is measured as an angle over one revolution, starting at 0 at pericenter/perigee, with 180 at aphelion/apogee.

Epoch: When the orbit is defined

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.

Additional satellite-related elements

Decay Rate describes the effect on a satellite when it collides with particles in the Earth's upper atmosphere. This friction causes satellites to eventually spiral downward, and as they do, they speed up. The Decay Rate orbital element simply tells us the rate at which Mean Motion is changing due to this drag or other related effects.

Its units are in revolutions per day per day (meaning every day, the satellite gains so many revolutions per day) and is typically a very small number.


Orbit Editor Hints

Sources for orbital elements

Orbital elements for comets, asteroids, or satellites can be found in magazines such as "Sky & Telescope" and "Astronomy", or on the World Wide Web. Check Sienna's web site for links to sites which contain recent orbital element data.

Converting dates for use in the orbit editor

The Orbit Editor requires you to input dates as Julian dates or NASA epoch dates. If the original date is in "ordinary" format, there is an easy way to convert it to a Julian date or NASA epoch. Start by plugging the ordinary date into the Time floater, then hit the Set Julian button. Copy the Julian date from this dialog box, cancel the dialog, then go back to the Orbit Editor and paste in the Julian date from the clipboard. To find the NASA epoch, switch to the AMSAT style, and the date will automatically be converted 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, then switch styles to Pericentric. Copy the Julian date from the Epoch data box, then press the Julian button on the Time Palette, paste in the new Julian date, then set the time. The Time Palette will then display the Julian date as a normal time.

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 these sliders to enhance the user experience.

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.

Working with satellites close to the Earth

On slower computers, if you are creating a satellite close to the Earth, you may wish to increase performance by turning off detailed surfaces and phases in the Settings/Options/Planets menu.


| Contents | Previous | Next | Frequently Asked Questions |
Page last modified on: January 25, 1999