Why certain days in the fall and spring are great times to go out and look at your satellite dishes

There is something that happens in the spring of every year, and then again in autumn.  I like to call it the “total eclipse of the satellites”, although that’s probably not a technically correct description because the satellites actually move in front of the sun, as viewed from the earth.  And of course, the satellites are nowhere near large enough to block the light coming from the sun.  But what is interesting is that on those specific days, at certain specific times, the sun closely tracks the satellite arc.  The exact days and times will be different depending on where you live – in the northern hemisphere, the further south you are, the earlier it will happen in the fall, and the later it will be in the spring.  When the sun is directly behind a satellite, it causes two things to happen:

First, for the time the sun is behind any given satellite, the solar radiation is much stronger than the signal produced by the satellite.  Therefore, you will lose reception for a few minutes on each satellite in turn, as the sun moves across the arc.  There is really nothing that can be done about this, and broadcasters often provide alternative feeds on other satellites or at different times precisely for this reason.  It may also be a reason that so many syndicated shows are fed in the middle of the night or the early AM hours, since they would then not be susceptible to this twice-yearly interference.

But second, there is also good news for satellite dish owners – the time when the sun is directly behind a satellite is a great time to check for obstructions that might be preventing you from receiving all the signal that your dish could collect, or for finding the optimal spot to place a new dish.  The trick is to find out the day and time when the sun and the satellite you are trying to receive are at their closest position in the sky – in other words, when the sun is directly behind the satellite, or very nearly so.  Then, you look at your dish to see if there are any shadows in it.  If there are, you determine what is causing the shadows and then determine whether you can remove that obstruction (by cutting a tree limb, for example).  If not, it might be a good idea to reposition the dish to some portion of your yard where there are no shadows present.  Note that on a smaller dish such as the type used for Ku band reception, something as small as the shadow from two or three tree leaves can cause a significant loss of signal.

It naturally follows that you can also use the “eclipse of the satellites” to find a place to position a new dish.  On the day and time when the sun is directly behind the satellite, look for a large open sunny spot and put your dish there.  If your dish will be on a pole, hold some object (it could even be your hand) at the approximate height of the center of the dish and try to get the shadow of that object to appear in the center of a large sunny area (an area larger than the size of the dish, at least), then plant your pole there.  Of course when you are doing that, be aware of any nearby shadows caused by objects that might sway considerably in high winds (enough sway for that object to come into the path of your satellite signal) – you want to avoid those if possible.  For example, locating a dish in a small sunny hole between a couple of large weeping willow trees might be a bad idea!

It could happen that the day that the sun is most closely behind the satellites is an overcast day.  That often happens at this time of year in some parts of North America.  It is okay to do this check a couple days before or after that day, as long as you do it at the exact time of day that the sun is closest to the satellite.

The real trick to this is knowing the exact date and time when the sun will be behind the satellite you want to receive.  To determine that, you need to know the exact latitude and longitude of your location, and you also need to know the location of the satellite in the sky.  All of these are expressed in degrees.  The satellite positions are easy to find; you can get them from just about any reference that shows the various satellites, such as Lyngsat.  As for your latitude and longitude, go to maps.google.com and zoom in on your house and yard – your latitude and longitude will appear in the address bar.  For example, if you were to see something like


That means that your latitude is 50.0106762 degrees (North if in North America) and your longitude is 110.112661 degrees (West if in North America).  By the way, that is actually the location of an interesting natural geomorphological feature, but I digress.

Now that you have those values, you can go to a site such as this one (requires Java):

Use sun or moon to choose the best place for your satellite antenna

Or this:

Sun Outage / Sun Interference Prediction for Geostationary Orbit Satellites

These sites will show you when the sun is nearest the satellites, provided you enter the required values correctly.

Note that if you are planning to install a movable dish, you can still check certain specific satellites that you are interested in, but what you want to do is find is a location where it is sunny all day on the day the solar outages are at their peak, or at least during that part of the day when the satellites you want to receive are behind the sun.

When checking for shadows on the dish or at a proposed location for a dish, be sure to check at the EXACT time the satellite is closest to the sun.  Most everyone has a cell phone nowadays that displays the time, so getting the precise time is not as much of an issue as it used to be.  But it really is important to look at the exact time, since the sun moves one full degree in four minutes!  Don’t believe it?  Well, the earth is divided into 360 degrees of longitude (180 degrees east to 180 degrees west) and 4×360=1440, which is precisely the number of minutes in a 24 hour day (6ox24=1440).  And a one degree difference is enough for the shadows of some obstructions to move off the dish.  If you do happen to be a minute or two late, look on the ground next to the shadow of the dish and see if there are any shadows very close to the edge of the dish.

With a larger C-band dish, a small obstruction such as a few leaves won’t matter as much, but be aware that trees normally don’t stop growing until they are quite large, so if you see the shadow of a few leaves creeping onto your dish, perhaps the time to do something about it is while the branch is small.

I will note that is is also possible to use the moon for this purpose, but typically that is not as accurate because it is rare for the moon to be directly behind a satellite.  There are also programs you can download to your phone, some of them free, where you can point your phone’s camera at the southern sky and in theory it will overlay what you are seeing with the locations of various satellites.  Here is a short video describing one such app:


While apps such as this can be very helpful, in my experience they are not all that precise.  If you are trying to receive a signal through a smaller hole between trees, having your dish two or three feet to the wrong side might be the difference between no obstructions and having maybe a third of your dish or more obstructed.  For finding a precise location in a tight area, or discovering if that stupid bush that your neighbor planted that’s grown like a weed is really interfering with your signal, there’s nothing like using these “eclipse days” to get an actual, visual representation of what is or isn’t causing interference with the satellite signals.

EDIT: Yet another approach is to use Google Maps, but this works best in areas where their satellite imagery is fairly sharp and clear. Just go to Dishpointer.com and you’ll be asked to enter your address and select the satellite you want to receive from a dropdown. But here is the interesting part, in many areas you can zoom into your yard and see quite a bit of detail. You can click on the green pointer and drag it to the location in your yard where you’d like to put a dish. It will show you the path to the satellite as a green line, and if that line doesn’t intersect any trees in your yard or your neighbors’ yards that’s a good indication. But unless you live in a desert or next to a lake, sooner or later that line will probably intersect trees. So you still need to determine if the satellite will be over the top of those trees. Under the Google Maps image, that site will also show you the elevation of the satellite – the higher the elevation, the closer the tress can be without problems.

A somewhat similar site, with the long title Google Maps based satellite uplink/downlink site planner with dish alignment look angles and topographic coverage ( clear line of sight calculation ) shows a similar Google maps view, but with the additional feature that if Google Street View has been through your area it will try to show you where the satellite would be in relation to the nearest picture taken by the Google Street View car, which may or may not be helpful.

Either of those sites will tell you the elevation of the satellite at your location, and the azimuth, which is its position in the sky relative to your location. So once you have that information, if you can’t wait for the next solar outage event, you could go to a Sun & Moon Position Calculator, enter your location or find your yard on the map, and scroll down to see where the moon is in relation to you (click the button to start the Autoupdater). The fields of interest to satellite dish installers are the bottom three: “Satellite Longitude (The sat-longitude which gives same azimuth as the Moon)”, Satellite Elevation, and “|Moon Elevation-Satellite Elevation|”.

This may be a little difficult to understand at first glance, but let’s say you are wanting to find a place in your yard to place a dish that will receive a satellite at 97°W. You would have to enter your location at this site, then start the Autoupdater and watch the Satellite Longitude as it counts up to 97°W – if it’s already beyond that, try at an earlier time the next day. When it reaches that location, see if you have a clear view of the moon in the sky from your proposed dish location, and at the same time also note the value in the “|Moon Elevation-Satellite Elevation|” field. This tells you how much higher or lower the moon is in the sky compared to the satellite. An ideal situation would be if it’s within a degree or so of the satellite elevation, but if the satellite elevation is a concern then it may help you estimate how much variation there is between the moon position and the satellite position.

Of course using the moon isn’t always easy, since it’s more easily obstructed by clouds than the sun, and also it rises and sets at different times of the day or night, and can be difficult to impossible to see on a bright sunny day, particularly if it’s only a partial moon. Many weather sites will tell you when the moon rises and sets in your location, though you may have to search a bit to find that information on the site, or you can use this Sun or Moon Rise/Set Table for One Year (be sure to select moonrise/moonset in the dropdown, and don’t forget to “Add one hour for daylight time, if and when in use”). If, on any given day or night, the moon is below the horizon (between moonset and moonrise) when it’s at the same longitude as the satellite as the satellite you’re interested in, you won’t be able to use it that day or night. And also, during the monthly dark of the moon (the day or two just before the new moon), it may be difficult to impossible to see in the sky.

I mentioned above the site Use sun or moon to choose the best place for your satellite antenna but since that requires Java, it won’t work on many modern computers (and personally I don’t recommend installing Java). If, however, you do have Java installed, and can figure out how to use the site, it can help you predict when the moon will be nearest (in terms of relative position in the sky) to the satellite you’re interested in.


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