Neptune Viewer Help

This form enables you to generate a diagram showing the appearance of the Neptune system at a specified time. All bodies are rendered with terminators and shadows as appropriate. Selected background stars can also be included.

In the diagram, Neptune and the moons are modeled as triaxial ellipsoids, and are drawn with latitude and longitude contours at 15 degree intervals. Illuminated regions are indicated with black lines; unilluminated regions and terminators are shown as light gray. Penumbral shadows are not indicated. Optionally, the 0 and 180 degree meridians can be shown with a heavier line.

A selection of the rings is also drawn. The rings are shown as single lines in black if illuminated and in gray if shadowed or if the opposite side is illuminated. The Galle and Arago Rings are plotted as dashed lines if selected. All rings are assumed circular and coplanar. The Adams Ring arcs are highlighted with heavier line segments. The user may select between the three models for their motion. The locations and spacings of the arcs are as tabulated by Porco (Science 253, 995-1001, 1991).

The diagram is oriented with J2000 declination increasing upward and with right ascension increasing to the left. The frame has uniformly-spaced tick marks along each axis. The declination axis is labeled in degrees, minutes and seconds; the right ascension axis is labeled in hours, minutes and seconds.

Each diagram includes a caption that summarizes the key parameters used to generate it.

Limitations. The outer irregular satellites of Neptune are not shown.

Change History

1.0 (February 8, 1997): Original Neptune viewer on line.
1.0a (August 5, 1997): Bug fixed that prevented Larissa and Proteus from being displayed.
2.0 (February 1, 1999): Extended ephemeris time limits and added new option. Added a viewpoint option including Voyager 2 and parallax corrections for Earth-based observatories. Added alternative units (Neptune radii, kilometers, and the Voyager camera fields of view) to the field of view options. Added background star options. Added an option to suppress latitude and longitude lines, producing diagrams that are suitable as drawing blanks for amateur observers. Added distances and light travel time to output table.
2.1 (January 31, 2002): Added star name option for diagram center. Added hour/degree options for right ascensions.
2.2 (December 1, 2009): Updated the default ephemerides. To reduce confusion, we have removed the choice about what ephemeris to use.
2.8 (January 23, 2013): Renumbered for consistency among all Planet Viewer tools. Ephemeris updates.
2.9 (January 4, 2016): Ported to new server. Ephemeris updates.
2.10 (October 14, 2020): Ephemeris updates; updated name for "Hippocamp".

3.0 (July 5, 2021): Major new features:

Observation Time

The observation time (UTC) can be entered in a variety of formats. For example, the following all parse to 0:01:02 UTC on July 4, 1976:

If you want the gory details of how times are interpreted, click here.

Field of View

Enter the field of view of the drawing to be generated and select the appropriate units. Units can be seconds of arc (the default), milliradians, microradians, Neptune radii, kilometers (projected at the distance to Neptune) or the fields of view of certain spacecraft cameras.

Arc motion model:

You may also select between the three competing models for the motion of the Adams Ring arcs. The first two were described by Nicholson et al. (Icarus 113, 295-330, 1995). The third (default) appears to be more consistent with recent Earth-based detections.

Diagram Center

Four different methods of specifying the diagram center are supported. Click on the box to the left of the option you wish to use.

Body: The diagram will be centered on the location of the selected body.

Ring or orbit ansa: The diagram will be centered on the ansa of the selected ring or orbit. Specify the east or west ansa using the second box. Note that east is toward the left in the diagram.

J2000 RA and dec: The diagram will be centered on the specified pair of right ascension and declination coordinates. The first box is for the RA and the second box is for the dec. You may also specify whether the RA is in units of hours or degrees. Enter up to three values in each box, separated by spaces; these values are interpreted as degrees/hours, minutes and seconds, respectively. Any or all values can have fractional parts.

Star name: Enter the name of a star as it appears in the current list. The name must match exactly.


You may specify the point of view of the diagram. By default, the point of view is the center of the Earth.

Observatory: You may select from any viewpoint on the list.

Latitude & Longitude: If your desired observatory or location is not on the observatory list, you can enter its latitude, longitude and altitude in the three boxes provided. Latitudes and longitudes can each be specified by up to three values, interpreted as degrees, minutes and seconds. Longitudes can be specified either east or west.

If you wish to have an observatory added to the standard list, or to refine the coordinates of a listed observatory, email the necessary information to Mark Showalter.

Moon Selection

You can decide which moons to include in the diagram. At minimum, Triton and Nereid are included. Use the boxes to the left of the list to choose the smallest set of moons to include. Note that, whatever your choice, the moons appearing above it in the list will also be shown.

Ring Selection

You can decide which rings to include in the diagram. The Adams and LeVerrier Rings are always shown. Click on the box to the left to show the Arago and Galle Rings. Note that, whatever your choice, the rings appearing above it in the list will also be shown.

Background Objects

You have several independent options for including background objects in the diagrams. These options make it possible to render diagrams around the times of stellar or spacecraft occultations. Objects are marked by pluses and are labeled by name if the Moon & Star Labels option is activated.

Standard stars: Check the box to include any of a standard list of stars that happen to fall inside the field of view of the diagram. The Neptune Viewer does not access a star catalog; instead, it only plots stars from a finite list. To view the current star list, click here. This list is updated periodically at the request of the users; if you would like to have a star added to the list, email the necessary information to Mark Showalter.

Additional star: In addition to or instead of the standard stars, you can specify one additional star to be included in the diagram. Check the box to the left and enter the star's RA, dec, and name in the three boxes. You may also specify whether the RA is in units of hours or degrees. Enter up to three values in the RA and dec boxes, separated by spaces; the values are interpreted as hours/degrees, minutes and seconds, respectively. Any or all values can have fractional parts.

Other bodies:

Check each box to mark the location of the specified body or spacecraft in the diagram.


Enter a title for the plot in this box. It will appear centered above the diagram.

Moon and Star Labels

Optionally, the diagram will be generated with the name of each moon and star written above and to the right of its center. You may select the size of these labels in points, where a point is 1/72 inches.

Moon Enlargement

The diagram is rendered to scale, which means that some of the smallest moons may be very hard to see. You may enter a minimum plotted size for moons in this box. If a nonzero value is used, the smallest moons will be easier to see. The size is in units of points, equal to 1/72 inches. A value of ~4 may be appropriate.

Blank Disks

If you select this option, all latitude and longitude lines will be suppressed in the diagram. This produces a diagram that is suitable for amateur observers to use as a drawing blank. This option is not available for spacecraft-specific viewers.

Arc Weight

Enter the width of the line segments to use to highlight the ring arcs, in units of points (= 1/72 inches).

Highlight Prime Meridians

If you select this option, then the 0 and 180 degree meridians will be shown with a heavier weight.


By default, when you click on "Generate diagram", you are directed to a web page that lists the details of the request, a small preview of the diagram produced, and options to view or download that diagram as PDF, JPEG, or Postscript. Optionally, you can choose to bypass the web page and go directly to the diagram in one of the three formats.

Neptune Viewer Form | Node Tools | Ring-Moon Systems Node Home

Last updated July 5, 2021

Mark Showalter