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THE INTERNATIONAL MARSWATCH ELECTRONIC NEWSLETTER
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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: http://www.astroleague.org/marswatch/. Bert Stevens of the
Astronomical League continues to work heroically to keep your posted images,
ftp upload/download site, and other information updated.
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(1) 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:
http://marswatch.tn.cornell.edu/hst99.html
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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(2) Latest A.L.P.O. Mars electronic newsletter
The Martian Chronicle: Newsletter of the International Mars Patrol
An Observing Program of the Association of Lunar and Planetary Observers
Daniel M. Troiani
ALPO Mars Section Coordinator
E-Mail: dtroiani@triton.cc.il.us or dantroiani@Earthlink.net
ALPO Home Page: http://www.lpl.arizona/~rhill/alpo
May 1999
Martian Seasons
by Don Parker
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 deg.. 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 degrees 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 (s. 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 degrees from the heliocentric longitude (h) listed for
that date.
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Other Useful and Related WWW sites:
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Latest MGS images:
http://www.msss.com/mars/global_surveyor/camera/images/index.html
Main MGS Home Page:
http://mars.jpl.nasa.gov/mgs/index.html
Mars-98 Orbiter:
http://mpfwww.jpl.nasa.gov/msp98/orbiter/
Mars-98 Lander:
http://mpfwww.jpl.nasa.gov/msp98/lander/
Mars-98 MVACS Science Payload Home Page:
http://mvacs.ess.ucla.edu/index.html
New Millenium Mars Microprobe Mission (DS2):
http://nmp.jpl.nasa.gov/ds2/
Pathfinder Home Page:
http://mars.jpl.nasa.gov/default.html
JPL Mars Missions Page
http://www.jpl.nasa.gov/mars
Mars-01 and Mars-03 APEX/Athena Science Payloads
http://athena.cornell.edu/
A.L.P.O. Mars observations:
http://www.lpl.arizona.edu/~rhill/alpo/mars.html
Astronomical League:
http://www.astroleague.org/
1998-99 Marswatch ftp site and imaging highlights:
http://www.astroleague.org/marswatch
1996-97 Marswatch highlights:
http://mpfwww.jpl.nasa.gov/mpf/marswatch.html
Mars (and other) Educational Resources Page
http://marswatch.tn.cornell.edu/marsidea
MarsNet:
http://astrosun.tn.cornell.edu/marsnet/mnhome.html
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I will continue to maintain the email distribution list as well
as the various Cornell and JPL Marswatch-related WWW archives.
NOTE: I apologize if you are receiving duplicate copies of this mailing,
but I STILL can't seem to track down the email glitch that is causing this.
If you would like your name removed from the distribution list, please
send me an email.
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Jim Bell
Cornell University
Department of Astronomy
Center for Radiophysics and Space Research
402 Space Sciences Building
Ithaca, NY 14853-6801
phone: 607-255-5911; fax: 607-255-9002
email: jimbo@marswatch.tn.cornell.edu
WWW: http://marswatch.tn.cornell.edu
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