Hands-free smartphone without a tripod to record the eclipse
This method involves use of specially-made wedges, or blocks of wood, books, or other objects, to place the smartphone on the ground, propped at the right angle to image the Sun during the few minutes around central eclipse. IOTA does NOT recommend this procedure, which is simple in principle, but that we find, in practice, is difficult to implement accurately enough. IOTA recommends instead, hand-holding the recording smart phone, as described here [give link to SmartPhoneSimple.docx page], or even better, mounting the phone on a photographic tripod described here [give link to SmartPhoneTripod.docx page], (some general information given on those other pages is not repeated here); both of these methods allow you to see the eclipse on your smart phone, so you know in real time that you are recording it, while with this method, you’re in the blind; you don’t know if you were successful until reviewing the recording after the eclipse, and if your pointing wasn’t accurate enough, the eclipsed Sun probably won’t be in the field of view of your recording. If you try this technique, you must practice it on one of the 4 days before the eclipse, at the same time as the eclipse, when the Sun will follow a path in the sky very close to what it will be during the eclipse, and of course, with exactly the same setup (wedge or other prop) at exactly the same place where you will observe the eclipse. If the practice recording doesn’t show the Sun, you should repeat, changing the prop angles, until a practice run records the Sun. Pointing to the necessary accuracy will be considerably more difficult if you use a smart phone telephoto lens (which we prefer that you do, in order to record the eclipse, and the Baily’s beads, in more detail), since the telephoto lens has a narrower field of view. One experienced observer plans to try this, with automated recordings with multiple smart phones, but he plans to construct special mounts that can be pre-pointed accurately enough.
A YouTube video shows how to record the eclipse using a smart phone placed on the ground without a tripod, but it has some general information that is covered better in (and superseded by) the later information here [link to SmartPhoneSimple.docx Web page]. There is another YouTube video showing the recording that was made with the smart phone “B” used in that video. As noted previously, you can ignore the part about imaging the geographical coordinates with the smart phone app, if you just take a couple of still pictures of your observing site with location tagging enabled.
This alternative approach, without using a tripod, is to just put your cell phone down on the ground and let it record by itself. But it needs to be pointed at the Sun, so I will describe how to set your phone at the right angles to record the Sun. You should test this procedure some day between Aug. 17 and 20, when the Sun will be in the same position in the sky, to good-enough accuracy, at the Aug. 21st time of central eclipse for your location. If you center the Sun two minutes before central eclipse, the Sun will stay in the field of view for the next 3 minutes that are needed. This is the case for low magnification, but if you use a smart phone telephoto lens, you should test to see how long the Sun stays in its field of view. Due to the Earth’s rotation, the eclipsed Sun will move from left to right across the field of view, so if you place the Sun on the left side of the field of view at the start, it will stay in the field of view for the longest time.
For placing phone “B” on the ground to record the eclipse, it needs to be pointed in the direction of the Sun, using its known angle above the horizon (altitude) and direction for your location. In order to use those angles, you need to be on a level surface. Carpenters are usually careful about making decks level, so they make good surfaces to use. But if you are on a sidewalk, it might not be level enough, if the road is on a small hill. Use a level device to check it; if necessary, use a flat surface, such as a card table with legs folded in, a piece of plywood, or even a large piece of stiff cardboard, cut from a large box, and place some magazines or other objects to prop up the downhill side, until the surface is level.
The first angle is the tilt angle to the horizontal; it’s called the “zenith distance” and is 90 deg. – the altitude of the Sun above the horizon. For the video demonstration, I used 64 deg. for the altitude, which is the maximum altitude of the Sun above the horizon in the eclipse path, from Missouri to western Kentucky. Then the tilt angle is 90 deg. minus the altitude, or 26 deg.
The direction, which will generally be towards the south, is called the azimuth. The azimuth of directly south is 180 deg. Many cities and rural areas have a grid of north-south and east-west streets; most in Kansas City are that way, but not in St. Louis city. For my video, made in early July in Maryland, the Sun’s azimuth was 123 deg. when the altitude was 64 deg., at 11:40 am EDT. So for the demonstration, I used the angle 180 deg. – 123 deg., or 57 deg. east of due south. I used a protractor, to plot these angles (26 deg. and 57 deg.) on a piece of paper that can be cut for positioning the smart phone.
If you live in an area with north-south streets, you can just use the curb to tell where due south is, but here in Maryland, the streets are in different directions. I tried to use the compass function of the free “GPS Test” app on my Android to find the north-south direction, but it was not accurate enough; I’ve found that smart phone direction apps generally use magnetic compasses that are not accurate enough. I knew from previous observations that the long board on our deck is aligned with azimuth 205°, or 25° west of due south, so I placed a piece of paper on our deck in that direction, so that its edge is true north-south. Then I use the 57-deg. paper wedge, to determine the direction of the Sun, and then I used blocks of wood, to prop up the phone to the proper tilt angle, using the 26-deg. cardboard wedge to determine that. Depending on how slippery the surface is, you may need some rocks or another fairly heavy object to stabilize the propped-up smart phone. The rocks, or other objects, may be needed to hold down the north-south piece of paper, and the 57-deg. wedge, especially if there is any wind. Sometimes, on a calm day, an “eclipse wind” will suddenly form as the air temperature falls as the eclipse deepens.
If the streets are not north-south in your area, use Google Earth to view the streets in your area, and select the street that you plan to observe from. Then use Google Earth’s ruler to draw a line between any two points on the edge of the street. The ruler dialog box not only gives the distance between the two points, but more importantly, the azimuth, or bearing, between them. From that, you can calculate the angle of the street from due north. It is better to use that than any compass, or compass app on a smart phone, to determine the direction. With the high altitude above the horizon of the eclipsed Sun, the pointing is fortunately relatively insensitive to errors in the pointing direction (azimuth).
David Dunham, International Occultation Timing Association, dunham@starpower.net .