1998-1999 Apparition

Linking Amateur and Professional Mars Observing Communities.

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The International MarsWatch Electronic Newsletter

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:

Also, check out plans as they are evolving for the NASA Mars 2001 Lander and Orbiter missions at:

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 Appoarition 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:

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 1998-1999

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° Ls). Mars will reach opposition on 24 April 1999 (128° Ls) and be closest to Earth on 01 May 1999 (132° 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° as compared to 23.5° 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° 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°/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.

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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 Next MarsWatch Newsletter (Volume 3; Issue 6; October 26, 1998)

Read the Previous MarsWatch Newsletter (Volume 3; Issue 4; April 27, 1998)

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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 January 17, 1999.