Re:[Az-Geocaching] Never MInd!

     Some of us have to sleep in the daytime after staying up all night 
observing, so I didn't get your message until now....  Here's the scoop on 
Mars and its "oppositions".  Opposition for any object orbiting the sun as 
viewed from Earth happens when the Earth, the sun, and the object (asteroid, 
planet, comet, etc) are lined up with the object farther from the sun than 
Earth (the inner planets do not reach opposition).  When this happens, 
several things of interest happen.  First, the object is as close as it can 
be in that time period (more on that in a bit - it might get closer at other 
times...).  Closer means brighter, and closest usually means brightest. 
Another affect happens that makes an object near opposition even brighter and 
that is the so-called "opposition affect."  An object exactly opposite the 
sun scatters light straight back towards the sun more efficiently than in 
other directions.  You can see this affect from airplanes or when the sun is 
low - you will see a bright halo around your shadow - around the planes 
shadow if you are sitting at a window where you can see the shadow or around 
your head if the sun is low and you're not on a plane.  I've seen this best 
on a golf course early in the morning when there is dew on the grass.  So 
asteroids, planets, the Moon, etc., will be that much brighter.  Also, the 
object will be full (like the full moon) rather than gibbous or quarter or 
crescent - that makes a big difference for objects close to Earth, but almost 
no difference for the distant planets, since they are almost always very 
close to full regardless of where they are in their orbits.
     Ok, I've probably lost a few of you.  I'll try to loose a few more of you 
in the next paragraph.... ;-)  I'm trying really hard to think of a way to 
bring geocaching into this, but I haven't thought of a good way yet....
     Back to Mars (well, I'd like to go there someday anyway...).  The orbits 
of the planets are all nearly circular, but they are very slightly elliptical 
which means that they change their distance to the sun as they orbit the sun. 
Earth's orbit is slightly elongated as well and at the moment we are actually 
pretty close to our most distant from the sun (the seasons are dominated by 
the tilt of the rotation axis with distance from the sun falling a very 
distant 2nd).  Mars orbits the Sun every 1.88 years and comes back into 
alignment with Earth every 2.14 years, approximately (this is called the 
synodic period and does not equal Mars' orbital period because the Earth 
orbits as well).  The closest Mars and Earth could get to each other would 
happen if Mars came to opposition when the Earth was farthest from the Sun 
while Mars was closest to the Sun.  This doesn't quite happen, but it is not 
far off.  The closest the two orbits get to each other happen in the 
direction that Earth is at the end of August when the two orbits are about 
34.4 million miles apart.  So, when Mars comes to opposition at the end of 
August, it will be as close as it can come and Mars will appear at its 
largest.  When it comes to opposition at the end of February, its opposition 
distance will be as large as it can be (around 65 million miles) and Mars 
will appear smallest at opposition (it can appear even smaller when observed 
away from opposition).  The opposition you discovered happened on about 
August 27, 2003 and it was VERY spectacular.  I got to look at Mars through 
the 90 inch telescope on Kitt Peak at about that time and Mars was very very 
big and bright.  I don't expect to see Mars with my own eyes any better 
unless I'm about to drop into orbit around Mars itself.  This year, Mars will 
be at opposition on about October 29 when it will be about 43 million miles 
from Earth and will appear about 20 arcseconds across (the Moon is about 1800 
arcseconds across for comparison) - not too bad as Mars oppositions go, but 
not quite what it was in 2003.  You can already see it in the morning sky - 
it's a bright red object all by itself about halfway up in the East before 
twilight starts.

This message is part of the following thread:
	the complete thread tree sorted by date
	Trisha Brasher at
	Jim Scotti at