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Unknown Cache

South Pole Iridium Flares

A cache by Iowa Tom
Hidden : 10/10/2005
In Iowa, United States
1.5 out of 5
1 out of 5

Size: Size: micro (micro)

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Geocache Description:

This cache is one that I hope will teach people about magnificent lights in the sky called “ iridium flares.” They are a reflection of sunlight from an antennae panel on an iridium satellite. They can be a sight to behold! Part of the fun here is using only what's at the cache site to retrieve the log.

The instructions for finding the real coordinates for this cache are pasted in a post that I dated then submitted for the earliest day that posts began coming in for another one of my geocaches. You should easily recognize it when you see it. To know what the name of the cache is that contains that post you will have to find various letters on this webpage that are colored sienna (brown). They are barely visible as being a different color on your computer screen. They're in the order of the name of the cache that contains the note. You may have to refer to the list of my caches to familiarize yourself with all the possibilities. That list is here. I found that I could best see the colored letters by copying all the text in the long description on this page. Then I pasted it into a Word document. I magnified it up to 200% and scanned each line until I found enough letters (ending with a punctuation mark) to match the name of a cache I set up.

When you find the cache please take care to put everything back in place. The cache itself is out of sight. You’ll have to figure out how to get it out of its home. A long middle finger may be needed to put it back properly. To get the log out you may have to use the nail that should be stuck through the 2 by 4. I am trying out a strip of Tyvek for the log. for the log. Ball point pen writes on it well but pencil does not as well. The Tyvek is still in a loop (from the envelope I cut it out of). Please sign only your initials and date in one of the boxes. When putting the lid back on please be careful not to cross thread the aluminum cap. The container I got at a Casey’s gas station for about 5 bucks. A picture of it is shown at the bottom of this page.

Iridium satellites are part of a fleet of more than 66 operating satellites, built for worldwide wireless communications. The satellite constellation consists of 6 planes with 11 satellites each (and some spares). Hence, an Iridium satellite passes at by in an approximate plane every 8 minutes.

The iridium cell phone system is unique in that it has no need of cell phone towers. Therefore a person can call from anywhere in the world. The cell phone signal is transmitted directly to the satellite and back again.

Each satellite has three bright silver flat panels upon which are mounted numerous box-like antennas. The panels are about 3 feet wide by 6 feet long. See one flaring here. They are a bright mirror smooth silver that can produce a very bright reflection of the sun’s light. The reflection (quite transitory) is called a flare, or specifically in this case, an iridium flare. They are bright enough to be easily seen from many hundreds of miles away. Just how bright do they get? Let’s compare the brightest flares to Venus, the evening and morning star that shines so brightly some times. The brightest of these flares get more than 90 times brighter than Venus ever gets! Ones that bright are more than a little noticeable in a dark sky! I read that one person who saw one without knowing what it was thought it was an airplane on fire.

The brightest ones appear as a light that gets brighter and brighter and brighter then dimmer and dimmer through a time span of about 10 seconds in a dark sky, about 3 seconds in a twilight sky and about 1 second in a blue sky. This time exposure of an iridium flare gives some idea as to how they brighten then dim. It was taken by Bjorn, a master satellite observer living in Europe. The iridium flares seen in full daylight are a challenge to see because they are not much brighter than the sky itself and you have to be looking at just the right spot to see it. I once wrote a story about a special daylight observation I made. I sent it to a satellite observing group I belong to. To easily find it for this webpage I did a Google search using the words, tom + driveway + iridium. It can be found here.

Truly spectacular, once I saw the light from a really bright iridium flare sweep as a faint light across a high thin cloud before I saw the flare itself. That was neat!! Few people have been fortunate enough to see that happen.

You can plan to see iridium flares yourself because they are very predictable. Heavens-Above (H-A) is a user friendly free service that does a good job of supplying information about when to look for them as well as other orbiting satellites. The H-A iridium flare prediction for the next 7 days going through the area where this cache is hidden is located at this site. The H-A main page for this cache site is located here.

It’s easy to edit your coordinates manually for any location then make a shortcut or save the URL so you can refer to it later. You don’t have to add in your altitude. It is not considered anyway.

On October 9, 2005, in the time frame in which I was working on this cache I saw that a magnitude -7 flare was passing the location I picked for this cache. I drove out to the cache site and captured it with my digital camera in spite of a whole host of problems that came up, one of which was a car driving by on the very dusty road two minutes before the event! That image is what you see below. The exposure was f2.8, ISO 100 and about 30 seconds long.

Satellite (and stellar) brightness is measured in magnitudes. It so happens that negative ones are brighter than positive ones. [Go figure.] If I see that an iridium flare is com’n through town, I myself don’t bother looking unless they are at least a magnitude -7. The -8 ones are real eye grabbers!

Iridium satellites are hurtling along at many thousands of miles per hour; about 16,710 mph to be exact. That figures to 4.6 miles per second! I determined that the flare that I saw on 10/9/05 moved 69 miles in 18 seconds. That figures out at 0.26 seconds per mile or 13,800 miles per hour. It was low in the north, hence the reason why the light swept by slower than what the satellite itself was moving.

An iridium satellite reflecting sunlight toward the Earth is pictured here.

Once I shared an iridium flare experience with Summit Dweller, a fellow geocacher. We set up a time when I could call him, cell phone to cell phone. My objective was to listen in on his first experience of seeing a magnitude -8 flare. I would see it first, but only as a -4 then after a few seconds he would see it but at -8! We were 36 miles apart. I will never forget hearing his kids yelling in the background about what they saw. “How bright is that gonna get?,” someone yelled.

They travel along a north-south direction. The width of the brightest part of the reflection is pretty narrow. Heavens-Above does tell you how far to drive and in what direction to see it at its brightest if you wish to do so. I use my GPSr to place myself on the centerline of the flare by finding one coord that will give what I want to see then changing only the latitude to make a second coord. I then use the two to make a single leg route. leg route. This is an example of the page that showed me where I would have needed to drive to get to the center of the path of the flare that I myself imaged. Here is a map showing the ground track traveled by the one that passed by me on 10/9/05. I modified the ground track image to show where the satellite was from me when I saw it flare. Notice that is was past the Canadian border at the time! I added a small section of a red line along the path) to show the place where the satellite was in its path when the flare moved past me. I also put a red circle over my observing site. This is a whole sky star chart showing the path of that satellite overhead at that time then here’s another and another chart closer still. In the last chart linked here you can see that I used a red circle to mark the location of the center of the flare.

These types of charts are all available at the H-A site. If anyone wants a step by step explanation about how I find these charts to make the exact flare position amongst the constellations obvious, just let me know. It would be best if I obtain an email address that I could mail an attachment to.

Now for the reason that I named this cache South Pole Iridium Flare. The story goes like this. In the spring of 2002 when I was using a planetarium program called TheSky IV (it shows satellite paths as well as stars) I noticed that over the South Pole all the iridium satellites crossed paths. That got me wondering if more flares would be visible there so I went to H-A to have a look. I discovered that on that particular day they had 176 flares! From midnight to 8:17 AM, 28 were magnitude -8 flares!! As it turns out, the closer an observer is to either pole, the more frequently they will see flares in a particular time when flares are occurring. As it turns out there are times at the Poles when they can have a -8 flare every 5 minutes!!

I went ahead and posted my discovery to the satellite group referenced earlier. Then low and behold, three years later an engineer that maintains scientific equipment at the Amundsend-Scott South Pole Station was seeing numerous bright iridium flares just as I had noticed would be seen. To learn more about them he did a Google search and came across the 3-year old archived post I made to SeeSat about their frequency at the South Pole. He (Dana) wrote directly to me to inquire about my 3-year old post and we’ve been corresponding ever since! My mention of the inquiry email I got from Dana can be found at this location. He began getting lots of pictures of the flares against a foreground of observatories and other structures in the camp. Many were in the aurora that is frequently seen in that location. His published discussion and images of these flares begins with a discussion at this part of his webpage. In the following months he got many pictures of them even though the temperatures fell to more than -100° F!

One of his cohort’s (Glen) got a remarkable time lapse sequence (with a still camera no less!) that shows an active aurora with flares occurring repeatedly as the satellites cruised by! See my favorite sequence by clicking on “open file” here. The green laser is monitoring the atmosphere. The flares begin to appear about half way through the movie as streaks. They came every 9 minutes.

If you decide to try to see a flare on your own I sure would like to see you post your sighting on this webpage as a note. If you get an image post that too. Happy geo-flaring!

I have decided to make Heavens-Above shortcuts to some cities where I think that some geocachers live. This is so that anyone in or near those places can quickly look to see if they are scheduled to have a bright iridium flare pass through their area. The first link listed is the city name followed by “Main Page.” That will lead to the H-A Main Page for that city. There a person can quickly look to see if the Space Station is going over and so on. The second link will guide them directly to iridium flare predictions for the next 7 days.

If anyone wants to see their city listed here please let me know. In Tom's Corner, at IGO in this forum, I occasionally announce what cities in the list below are going to see a bright evening flare.

Ames Main Page and 7 day forecast for iridium flares

Belle Plaine Main Page and 7 day forecast for iridium flares

Bettendorf Main Page and 7 day forecast for iridium flares

Cedar Falls Main Page and 7 day forecast for iridium flares

Cedar Rapids Main Page and 7 day forecast for iridium flares

Davenport Main Page and 7 day forecast for iridium flares

Des Moines Main Page 7 day forecast for iridium flares

Dubuque Main Page and 7 day forecast for iridium flares

Iowa City Main Page and 7 day forecast for iridium flares

New Virginia Main Page and 7 day forecast for iridium flares

Shellsburg Main Page and 7 day forecast for iridium flares

Toledo Main Page and 7 day forecast for iridium flares

Vinton Main Page and 7 day forecast for iridium flares

Waterloo Main Page and 7 day forecast for iridium flares

Winthrop Main Page and 7 day forecast for iridium flares

Military Time Local Time
12:00 12 PM
13:00 1 PM
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15:00 3 PM
16:00 4 PM
17:00 5 PM
18:00 6 PM
19:00 7 PM
20:00 8 PM
21:00 9 PM
22:00 10 PM
23:00 11 PM

Additional Hints (Decrypt)

Na bqq cynpr sbe n cbfg

Decryption Key


(letter above equals below, and vice versa)

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There are no Trackables in this cache.



60 Logged Visits

Found it 37     Didn't find it 2     Write note 14     Temporarily Disable Listing 1     Enable Listing 1     Publish Listing 1     Owner Maintenance 4     

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**Warning! Spoilers may be included in the descriptions or links.

Current Time:
Last Updated: on 10/27/2013 1:16:34 PM Pacific Daylight Time (8:16 PM GMT)
Rendered From:Unknown
Coordinates are in the WGS84 datum