1998-1999 Apparition

Linking Amateur and Professional Mars Observing Communities.

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The International MarsWatch Electronic Newsletter

Volume 4; Issue 5
May 31, 1999
Circulation: 1514

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Dear Marswatch participant,

Two items in this latest installment of the newsletter:

  1. 1999 HST Mars images available online

  2. Latest A.L.P.O. Mars electronic newsletter

Also, remember that the 1998-1999 Marswatch Web Site continues to be a big hit. More than 1700 images and drawings from 39 contributors in 16 countries have been uploaded. To get to the site, point (and bookmark!) your browser to: David Knighton of the Astronomical League continues to work heroically to keep your posted images, ftp upload/download site, and other information updated.

--Jim Bell
Cornell University

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1999 HST Mars Images Available Online.

All of the Hubble Space Telescope (HST) observations of Mars obtained during the period around opposition are now posted online at the following URL:

The images were obtained in 4 "visits" between April 27 and May 7. We had some glitches with the file headers, causing some delays in processing the images, and we were also sidetracked by the excitement surrounding HST's images of a huge polar cyclone in our first visit (for that story, follow the link on the above Web page). The images that are posted online now have been processed for instrumental artifacts (bias, flatfield, cosmic rays) at a preliminary level only. We plan to update the images later this summer, once we find time to run them through the full processing procedure. Nonetheless, the images are quite spectacular, and hopefully will prove useful to Marswatch observers.

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The Martian Chronicle: Newsletter of the International Mars Patrol
An Observing Program of the Association of Lunar and Planetary Observers
May, 1999

Daniel M. Troiani
ALPO Mars Section Coordinator
E-Mail: or
ALPO Home Page:

Martian Seasons.

     By: Donald C. Parker, A.L.P.O. Mars Section

Ground-based observers of Mars have long realized that the planet, with an axial tilt similar to Earth's, has seasons characterized by cyclical changes in the sizes of its polar caps, appearances of some albedo features and behavior of its atmosphere. A particular martian seasonal date can be determined by knowing where the planet is in its orbit at that time. This position is the "Heliocentric Longitude" of Mars, which is a measure of Mars' angular distance in its orbit from an arbitrary "standard" starting point of zero degrees as viewed from the Sun's center. Heliocentric Longitude is represented by the Greek letter Eta (h). From observation one could determine that northern hemisphere summer on Mars starts when h is around 175°. This system was employed until the 1970s, when spacecraft data revealed that the martian axial tilt was somewhat different than previously measured. This meant that the martian seasonal dates had to be changed relative to Mars' orbital position. A more direct and meaningful method of describing the planet's seasons was employed, and this is the one that Mars observers use today. It is called Ls (pronounced "el-sub-ess.").

The term "Ls" refers to the planetocentric longitude of the Sun along the ecliptic in the planet's (in this case Mars) sky. By convention, 0° Ls occurs at the northern hemisphere vernal equinox, which is when the Sun, moving northward, crosses the Martian celestial equator. Ls=90° occurs at Martian northern hemisphere summer solstice (southern hemisphere winter); 180° at the northern autumnal (southern vernal) equinox; and 270° at the northern winter (southern summer) solstice. Mars is at aphelion at 70° Ls and perihelion at 250° Ls. Thus the planet is farthest from the Sun in late northern spring and closest in late southern spring.

Recently we have noted that many standard ephemerides no longer list Ls but continue to present h, probably because the latter value is more useful to those planning to send spacecraft to Mars. Since few of us in the ALPO Mars Section are planning trips to the Red Planet in the near future, we prefer that observers continue to use Ls. To calculate the Ls for a given date, simply subtract 85° from the heliocentric longitude (h) listed for that date.

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Jim Bell will continue to maintain the email distribution list as well as the various Cornell and JPL Marswatch-related WWW archives. If you are receiving duplicate copies of the International MarsWatch Electronic Newsletter, or you want your name added to or removed from the distribution list, please send him an email at

Jim Bell
Cornell University
Department of Astronomy
Center for Radiophysics and Space Research
424 Space Sciences Building
Ithaca, NY 14853-6801
Phone: 607-255-5911; fax: 607-255-9002

Read the Previous MarsWatch Newsletter (Volume 4; Issue 4; April 18, 1999)

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This page is maintained by David Knighton for the International MarsWatch. The 1998-1999 MarsWatch site it hosted by the Astronomical League as a service to the astronomical community. Comments, corrections, and suggestions can be addressed to This page last updated June 1, 1999.