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THE INTERNATIONAL MARSWATCH ELECTRONIC NEWSLETTER
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Volume 3; Issue 5
August 20, 1998
Circulation: 1464
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Dear Marswatch participant,
Enclosed is the second installment of the series of Marswatch emails
dedicated to preparations for the 1998-1999 observing season. The
series was written by Jeff Beish and other experienced Mars-observing
colleagues who are involved with the Association of Lunar and Planetary
Observers (A.L.P.O.).
Meanwhile, Mars Global Surveyor continues to return spectacular images and
other data during its orbit-lowering maneuvers. New images include
spectacular views of the Pathfinder landing site, "splosh" craters,
sand dunes, and some beautiful canyon and valley terrain. Check out
the images at either of these URLs:
http://mars.jpl.nasa.gov/mgs/index.html
http://www.msss.com
Also, check out plans as they are evolving for the NASA Mars 2001 Lander
and Orbiter missions at:
http://www.sun.com/mars/2001/index.html
Finally, more information will be provided soon about how and where
Marswatch images from the 1998-1999 apparition can be uploaded to and
downloaded from via a web and/or ftp site.
--Jim Bell
Cornell University
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1998-1999 APHELIC APPARITION OF MARS: PART 2
By: Jeffrey D. Beish
With Donald C. Parker, M.D., Daniel Troiani, and Daniel Joyce
THE MARS WATCH OBSERVING PROGRAM
The Marswatch program was initiated in electronic form in
1996 through the collaboration of astronomers at Cornell University,
the JPL Mars Pathfinder Project, and the Mars Section of the
A.L.P.O. as a vehicle through which Mars astronomers worldwide
can upload their observations to a WWW home page and archive site
at JPL.
MarsNet is the WWW arm of the International Mars Watch, a
group founded by professional astronomers interested in Mars to
facilitate better communication between the amateur and professional
Mars observing communities. At those Internet sites, you will find
images of Mars contributed by amateurs and professional, tools to aid
you in planning your own Mars observations, current and past issues
of the International Mars Watch Electronic Newsletter, and links to
other Mars-relevant sites on the Internet. The primary purpose of this
project is frequent CCD imaging of Mars using B,V,R or other
standard filters and visual drawings and photos in order to monitor the
planet's atmospheric dust and cloud activity.
Secondary goals include imaging or spectroscopic
characterization of the surface color and mineralogy, characterization
of the growth and retreat of the polar caps, and analysis of
atmospheric water vapor content. Because Mars rotates at nearly the
same rate as the Earth and it also has a dynamic atmosphere that
exhibits hourly, daily, and seasonal changes, frequent observations
from observatories spanning the widest possible range of longitudes
are desired.
The upcoming apparition (1999) is particularly important
because the U.S. Orbiter (Mars Global Surveyor) will start regular
imaging during this time. In addition, the orbiter will be in a low sun-
synchronous polar orbit, so it will only "see" the surface of Mars
around 2 a.m. and 2 p.m. local time (the rest of the planet is over the
horizon), so quality ground-based observations are needed in order to
place these single-time-of-day orbiter views of the planet as well as the
single-location lander data, into a global context.
The project will maintain a WWW home page and archive site like that made
in association with the Mars Pathfinder mission. The goal will be to
have participants submit one or more of their images (or entire data
sets if they like) to this site for dissemination to NASA Project
personnel, professional astronomers, amateur astronomers, news and
print media, educators and schoolchildren, and the general public.
Another general project goal is to post at least one new CCD image of
Mars on the Web every day between December, 1998 and December,
1999. Even better would be one "daily global view" per day,
composed of 2 or 3 Mars images taken on the same night but from
observatories widely separated in longitude. To make this a reality will
require a dedicated and geographically-diverse network of observers.
The current web site address for MarsNet, from which you can get to the
1996-97 Marswatch archive, is:
http://astrosun.tn.cornell.edu/marsnet/mnhome.html
Images from both the 1995 and 1997 apparitions may be viewed there. When it
becomes available, the address for 1999 will be announced in the
Martian Chronicle and on the ALPO Home Page.
MARS IN 1998-99
Mars has an average 15.8-year seasonal opposition cycle,
which consists of three or four Aphelic oppositions and three
consecutive Perihelic oppositions. The 1998-99 apparition will be
considered an Aphelic apparition because opposition occurs only 58
degrees after aphelion (70 deg. Ls). Mars will reach opposition on 24
April 1999 (128 deg. Ls) and be closest to Earth on 01 May 1999 (132 deg.
Ls) with an apparent diameter of 16.2 seconds of arc. Mars will be at
a distance of 0.57846 A.U or 53,771,107 miles from Earth.
For observers located in Earth's Northern Hemisphere, Mars
will not be positioned as favorably during the upcoming apparition as
it was in 1997, since it will be placed south of the celestial equator
throughout the entire apparition.
Mars' North Pole will be tilted earthward during the entire
1998-1999 apparition, permitting study of the planet's northern
hemisphere during Martian late spring, summer, and autumn. Thus
astronomers can again investigate the regression of the NPC and
follow Martian arctic meteorology. This apparition should also allow
careful scrutiny of the summer NPC remnant.
DAYS AND SEASONS ON MARS
The Martian solar day, also called a "sol" by space scientists,
is about 40 minutes longer than a day on Earth. Thus Mars rotates
through only 350 deg. of longitude in 24 hours. An astronomer on Earth
who observes a particular surface feature on Mars, on a particular
night, sees the same feature 10 deg. further to its west (closer to its
morning limb) the next night.
Mars and Earth have four comparable seasons because their
axes of rotation are each tilted at about the same angle to their
respective orbital planes. In describing Martian seasons, scientist use
the term "Ls" which stands for the Areocentric longitude of the Sun
along Mars' ecliptic. ("Areo-" is a prefix often employed when
referring to Mars or "Ares.") Mars' axial tilt is 25.2 deg. as compared to
23.5 deg. for that of the Earth. The Martian year is 687 Earth days, nearly
twice as long as ours, so that the Martian seasons are similarly longer.
While Earth's are nearly equal in duration, the length of a Martian
season can vary by as much as 52 days because of the greater
eccentricity of its orbit.
The axis of Mars does not aim at our North Star, but is
displaced about 40 deg. towards Alpha Cygni. Because of this celestial
displacement the Martian seasons are 85 deg. out of phase with the
terrestrial seasons, or about one season in advance of ours.
Consequently, when you observe Mars next spring and summer you
will be seeing summer and autumn, respectively, in the Martian
Southern Hemisphere.
MAKING OBSERVATIONS OF MARS
The ancient art of visual observation at the telescope is still a
most useful tool to the modern astronomer, and is the forte of the
amateur astronomer. The authors, attending various professional
meetings over the past few years, were pleasantly surprised to find
that carefully made amateur drawings were considered to be useful
sources of data by Mars professionals.
Even at its best, Mars is challenging to observe. The disk is
tiny and its markings are blurred by the Earth's atmosphere. A
telescope for planetary work should provide sharp images with the
highest possible contrast. A long-focus refractor is generally
considered the best, followed by a long-focus Newtonian or
Cassegrain reflector. Telescopes with large central obstructions do
less well.
Anyone who observers Mars will find it rewarding to make a
sketch of whatever is seen, both to create a permanent record and to
help train the eye in detecting elusive detail. Start with a circle 1-3/4
inches (42 mm) in diameter. Draw the phase defect, if any, and the
bright polar caps or cloud hoods. Next, shade in the largest dark
markings, being careful to place them in exactly the right locations on
the disk. At this stage, record the time to the nearest minute. Now
add the finer details, viewing through various color filters, starting at
the planet's sunset limb. Finally, note the date, observer's name, the
instrument(s) used, and any other relevant information.
The Martian Central Meridian (CM) is an imaginary line
passing through the planetary poles of rotation and bisecting the
planetary disk and is used to define what areographic longitudes are
present on the disk during an observing session. It is independent of
any phase which may be present--if Mars presents a gibbous phase the
CM will appear to be off center. The CM value is the areographic
longitude in degrees which is on the central meridian of the disk as
seen from Earth at a given Universal Time (U.T.). It can be calculated
by adding 0.24/min., or 14.6 degrees /hr., to the daily CM value for
0h U.T. as listed in The Astronomical Almanac.
The terminator (phase defect) is the line where daylight ends
and night begins. The terminator phase, or defect of illumination, is
given in seconds of subtended arc on the apparent disk, or in degrees
(i) or the ratio (k), to define how much of the geometrical Martian disk
is in darkness. The sunset terminator appears on the east side, or
evening limb, before opposition; and after opposition, the terminator
becomes the sunrise line on the morning limb on the west side. At
opposition there is no perceptible phase defect.
The axial tilt. The declination of the planet Earth (De) as
seen from Mars defines the axial tilt of Mars relative to Earth. The De
is also equal to the areographic latitude of the center of the Martian
disk, which is known as the subearth point. The latitude is (+) if the
north pole is tilted toward Earth and (-) if the south pole is tilted
toward Earth. This quantity is an important factor when drawing
Mars or when trying to identify certain features.
=============================================
Part 1 of this series, an introduction and description of the A.L.P.O.
and International Mars Patrol observing programs, can be found at:
http://astrosun.tn.cornell.edu/marsnet/imw/imw3.4.html
Part 3 of this series, in the next newsletter, will include more details
on observing Martian surface features and Martian meteorology, as well
as a calendar of events during the 1998-99 apparition.
For more information, the A.L.P.O. WWW home page can be found at:
http://www.lpl.arizona.edu/~rhill/alpo/mars.html
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Other Useful 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
Pathfinder Home Page:
http://mars.jpl.nasa.gov/default.html
JPL Mars Missions Page
http://www.jpl.nasa.gov/mars
Mars-98 MVACS Science Payload Home Page:
http://mvacs.ess.ucla.edu/index.html
Mars-01 Athena Science Payload
http://astrosun.tn.cornell.edu/athena/index.html
A.L.P.O. Mars observations:
http://www.lpl.arizona.edu/~rhill/alpo/mars.html
1996-97 Marswatch highlights:
http://mpfwww.jpl.nasa.gov/mpf/marswatch.html
1996-97 Marswatch ftp site:
ftp://marsnt3.jpl.nasa.gov
MarsNet:
http://astrosun.tn.cornell.edu/marsnet/mnhome.html
Mars (and other) Educational Resources Page
http://marswatch.tn.cornell.edu/marsidea
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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. If
you are receiving duplicate copies of this mailing, or you want
your name removed from the distribution list, please send me email.
--Jim
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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
email: jimbo@marswatch.tn.cornell.edu
WWW: http://marswatch.tn.cornell.edu
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