Getting the Right Time

It sounds so simple ....

But this item alone may determine the difference between success and failure.

This app works by making predictions of when an eclipse is going to occur at a particular location. It then triggers the camera based on its knowledge of time. Your position needs to be accurate to within a kilometer ( ~ ½ mile). Time is pickier. It would be best to know time to about ½ second (Although the program has a built-in fudge that tolerates a slightly worse error).

Every smartPhone (at least the iPhones and various Androids) have an app that displays the internal GPS. Mac^®'s unfortunately do not have this. The LocationManager built into Mac^®s makes a number of assumptions that are valid in a city, but are not when your reference WiFi source is cruising in the middle of an Ocean. For those using this program on a ship I included the ability to determine your location via an external GPS .

So that leaves time. In my experience I have always had to make time adjustments at the site (which is pretty scary). That is why I have added a number of tools to make this part of the process more routine. Here they are in order of my own preference

Learning the Correct Time

GPS

I originally implemented GPS support for those that are attempting this on a moving ship. That still works, but after fiddling with Mac Time Sync (the next choice) in a hotel room in a remote location while packing for the site I have decided that my GPS accessory will be accompanying me in the future.

Originally I was going to support the NMEA standard and the location part of the GPS support actually does this. But ( and this is a big BUT) the standard does not allow for a reliable method of obtaining time. The reason for this is complicated, but if you want some fun reading here is the explanation. The uBlox 7 chip has extra features that solve this. More on this below

Mac^® Time Sync

The Mac^® sitting on your desk is probably always properly syncd. In my testing the difference between what the Mac^® reports and and the GPS are typically in the 0.05 sec range. Put your laptop on a plane to a different part of the world and keep it disconnected for a week or so and the time may not be as good. But, if the internet is available to you ( remember Oct 23 and Apr 24 are in the US not Zambia) then that provides a path that does not require yet another piece of equipment (although it involves a little system magic)

Open a Terminal window and type the following (what is colored)

Mac-Pro:~ me$ su {Admin User}
Password: {Admin Password}

The default interactive shell is now zsh.
To update your account to use zsh, please run `chsh -s /bin/zsh`.
For more details, please visit https://support.apple.com/kb/HT208050.

bash-3.2$ sudo sntp -sS time.apple.com
sntp 4.2.8p10@1.3728-o Tue Mar 21 14:36:42 UTC 2017 (139~6597)
kod_init_kod_db(): Cannot open KoD db file /var/db/ntp-kod: No such file or directory
2021-03-23 16:46:37.660098 (+0700) +0.00071 +/- 0.000628 time.apple.com 17.253.4.253 s1 no-leap

Ignore the error. That is for a log that is not required. The big news is that you are now sync'd to within 0.000628 seconds of the time reference. You don't have to use time.apple.com. Any of the published time sources will work just fine. Do notice the error number since if it is too large then the time update may not be that good. I would be particularly concerned if you are using a VPN or a ships' internet. If you are using a VPN keep in mind you want a time source close to the VPN outlet (e.g. if you are using a VPN in that connects to the internet in Australia then pick ntp1.anu.edu.au or au.pool.ntp.org which are not on the previous list).

Remember that if you have an iPhone and have cell coverage you can tell your Mac to use your iPhone as its wifi source. Thus you can do a final check and/or time sync while you are set up at the site.

Time Offset

The last alternative (and it really is a do this only as a last resort) is to use a smart phone connected to Cell Service as a time source. Cell Services publish a time to the connected phones. How accurate is it the 3rd world?!? Well in Chile it was pretty good and it will be pretty good in the US. But the last caveat is this requires manually transferring the time from your phone to the program. Fortunately the program includes a tool that will do this to the accuracy of your reaction time. See Offset Time below.

Verifying your Clock

Let's say you have done one of the actions above to set your computer's time. How do you know it is correct?. For that you can use time.gov. If you look at the circled black panel to the right of the page you will see your clock error displayed. If you do not have reliable internet service then you will have to fall back on using GPS as described below.

Real Time Modes

In the QuickStart you were introduced to the Practice submenu in the Time Menu. Now it is time to talk about the operational menu items.

System Time

This is the default, but its behavior changes depending on whether you have a GPS that is ready as a TimeRef"

No GPS

If you do not have a GPS that currently reports it can be a TimeRef then this is just the simple case of using your system clock time. In this mode it is important to follow the instructions in "Learning the Correct Time"

With a GPS

If you have a GPS that has listened long enough to become a TimeRef then what happens changes. The first time you run you will notice the run mode in the main window switches to "Use GPS Time". In this mode it does not matter how well the Mac was sync'd since the time used will be from the satellites. The GPS window will show the offset from GPS time to your system time.

If you are starting from a cold start your GPS may take as long as 15 minutes before it is usable as a time source. The unit must first receive a message from a satellite. Thus I strongly recommend plugging in the GPS as soon as you arrive on site. As long as it has power it is listening for satellites

Offset

This mode requires more explanation. In this case you know (assert) that the Mac^®'s system clock is wrong, but you have a time source (e.g. your iPhone) that is correct. As stated above this assumes your iPhone actually is correct and has not been on airplane mode for a week while you are on the ship.

OK you trust your iPhone
When you select the Time->Offset menu item you will see the dialog on the right (below). Think of a WW II movie where the commander says "Synchronize your watches". This is the same. Edit (if necessary) the displayed time to be a convenient time in the future for your time source. Watch your clock. At the exact moment the correct time is displayed there press Now!.

Using the GPS for Time

If you use a real GPS in the operational modes above, they will set your location to where you are (that is what you asked for) instead of either one of the canned locations or the location you intend to observe at. The ship simulation gets around this by simulating the GPS. But if you want to see what will happen if want to see what happens with using a live GPS as a time source it requires some trickery.

This video shows setting up the GPS to act as a time source. Before the video started I plugged the GPS in. The "eclipse" I am using is a fake eclipse that I coopted from one that happened in the 1930s by setting the time when it is going to occur to a time in the future.

Of course since I am sitting in my back yard I cannot accept the site definition because the program will just tell me there is no eclipse here.

It took about 2 minutes from cold start to 3-D fix. Getting the time reference signal can take as long as 15 minutes; however if the unit collects data whenever it is plugged in. Thus if you are outside and your laptop is on it the GPS is listening and the times will be shorter. Also the GPS does not forget if this program restarts.

Camera Time

Once again this seemed so simple...

One of my frustrations has always been that the time in the camera is never synced to the correct time nor has (reliably) the resolution to be useful. I found this again as I tackled this problem. Despite the camera claiming to have millisecond resolution, time stamps are only one second. And only a handful of cameras allow the program to set the time (although Canon written utilities can set it in a broader range of cameras.)

I provided a tool that allows you to set the time in those cameras that allow it and at least display the time in those that don't. The program will optionally try to set the camera time via a Camera Preference in those cameras that allow it.

However I do not have confidence that setting the time resets the entire counter.

Thus if you want to know when your picture was taken I suggest you use the log file. That records a time stamp immediately before the command to press the shutter. It has an accuracy of 0.1 sec and is as accurate as your setting of the run time. When the downloader downloads an image it sets the capture time to the time in the camera. I would treat these with suspicion and rely on the speed, iso, and fstop to match images to the log.