A History of Time: Part 1

It’s 10:39 AM on January 4th, 2019 as I sit and begin typing this in the Buckhead area of Atlanta.  Around the world, if other people were to read that (with appropriate language translations applied), there would be an overwhelming agreement as to how long ago that was. Such an agreement wasn’t always the case throughout human history.  Throughout time humans kept different calendars and clocks in different civilizations.  It hasn’t always been possible to take a statement like my first sentence and reach the same consensus on it that we do today.

My thoughts in this article are reflecting on the changes in time keeping methods throughout history and how they differ.  They are related to a simple question that someone asked me: how do I know what time the sun will set.  There are some other astronomy related programs that I plan to discuss in the future and will be referring back to this post when expanded information is needed on a topic.

Solar Days

Across human cultures, one concept of time measurement is derived from the apparent movement of the sun.  This is a frequent and common celestial observation that is shared among humanity in most areas on the earth that are populated.  Most of us regularly see the sun appear to rise above the horizon in one area of the sky and go back below the horizon in another. At areas close to the polar regions, depending on the time of the year, the sun might not go below the horizon, but instead travels in a circle around the observer (circumpolar movement).  The same areas at other times of the year might not see the sun, but they still have the experience of being able to make the celestial observation.

From this observation humans have created the concepts of the day and the night and can communicate about past and future events in terms of the number of instances of this observation. The first division of this period is the instance in which the sun is above the horizon and the times in which it is below the horizon (day and night) and the instances at which it is making this transition (sunset, sunrise). There is also the instance at which the sun is at its apparent highest point in the sky; solar noon.


In East Asia a day was divided into 24 periods. During one of these divisions the starts of the night sky appear to move about the pole by 15 degrees. (Note: you can use the word “hour” to talk about 15-degree increments absent the concept of time).  An even finer division of these periods is thought to have come from the Sumerians through the Babylonians. They had a counting system that instead of being base-10 was base-60.  The Sexigesimal.

Mean Solar Day

If you measure the passage of time from one sun rise to another you’ll find that it isn’t the same every day and wobbles throughout the year. This is in part from the earth’s orbit around the sun being eliptical instead of circular and part from the 23 degree tilt of the earth with respect to it’s orbit around the sun. The longest period between sunrises is experienced about December 21 or December 22 (the winter solstice) and the shortest is at the summer solstice about June 20. As it turns out the apparent movement of the sun isn’t a great way by itself to keep track of time. But we can take the average (mean) of the length of these periods over a year and use that as a measurement of time. This is the time that our watches and other time pieces are based on; the mean solar day.

Sidereal Day

Celestial Sphere

Relative to each other the stars appear to not move.  From one night to another they appear to be in fixed positions rotating around the earth. This is sometimes visualized as a rotating sphere on which lights are fixed known as the celestial sphere.

Solar observations are not consistent from day to day, but observations of the stars over a considerable period of time are consistent. your face towards the sun at noon on one day and then turned your face to the sun at noon at another day  (let’s say 180 days later) you would not be facing in the same direction. If you turn your face towards a star on one night and then turn your face towards the same star on another day both days you are facing the same direction. movements with respect to the starts are sometimes described using a Latin word for stars, sidera. An adjective form of this word is sideral (prnounced sahy-deer-ee-uhl)

Temporary Graphic. To be replaced with SVG

Unlike the sun, the stars are in the same apparent position for every 360 degrees of the earths rotation (at least if we are talking about periods of time of a lifetime or less). The earth completes a rotation every 23 hours 56 minutes and 4 seconds. This period of time is called a Sidereal Day.  But because a sidereal day is slightly shorter than a solar day on any given calendar date there are 3 minutes and 56 seconds of Sidereal time that occur twice within the same calendar day.

Sidereal time is especially useful in communicating celestial observations. Since sidereal time is related to the position of an observer a celestial observation that is communicated with a direction and a sidereal time can easily be interpreted as the same direction by someone else on the earth. With mean solar time more work is required to do such a conversion.

For Solar Meantime there’s a contemporary preference to using UTC (Universal Time ) instead of Greenwich Time. But since Sidereal time is relevant to a longitude any agreed-up sidereal timezone is inextricably connected to locations. So you will see Greenwich Sidereal Time communicated as one zone of sidereal time with there being a continuous number of Local Sidereal Times as one circumnavigates the earth. Sidereal times, like someone’s position, are continuous and not segmented into a discrete number of zones like solar mean times.


Local Time

In an era in which a considerable amount of time was needed to move from one populated region to another each area had it’s own local time. There could be variation in the agreed upon time from one region to another, but it didn’t matter much. Once faster forms of transportation became an option (in the form of trains) . It was necessary to coordinate actions in different areas in order to keep schedules.


The second brightest object in the sky, the moon, is also a the inspiration for another time division that is commonly used. Like the stars over the course of 24 hours the moon appears to move with the rest of the celestial sphere. But the moon also appears to move along the celestial sphere. One cycle of the moon moving about the celestial sphere is called a sidereal month. This period last 27.32166 solar mean days. One can also measure the moon’s change in illumination cycles driven by the changing geometry of the positioning of the earth, sun, and moon. The time for this cycle is 29.53059 solar mean days. This period is called a synodic month.  The moon goes through 12 complete  cycles in a year in addition to a fraction of a cycle. Constructing months based on the whole number of moon cycles will result in a calendar that drifts around the calendar.


The seven day week were followed by Judaism and Islam with one of the days being a day of rest. The Romans had an 8 day week with one of the days being for market day. But eventually roman astrologers assigned a wandering star (planet) to each day of the week. They named the days of the week after Saturn, the Sun, Moon, Mars,  Mercury, Venus and  Jupiter.


Progressive Web Apps in Chrome

Progressive Web Apps (PWA) are HTML based applications that run as though they are desktop applications.  Google Chrome received support for PWAs on Chrome OS in May with the release of Chrome 67.  Linux and Windows received support in August with the release of Chrome 70.  Support for Mac OS X is yet to come.

Download code (415 KB)


One of the first differences that stands out for PWAs is that they can run in their own application window and are indistinguishable from other applications running on a machine. That difference is largely visual. But the differences extend well beyond what is visible. Resources that are not usually available to an HTML page are available to a PWA such as access to Bluetooth, serial ports, UDP networking, and more.  Chrome PWAs can be installed and have their own icon in your programs menu and function offline.

There are requirements that must be satisfied before an HTML page can be installed as a PWA.  These are the conditions that must be met.

  • The page must be served over SSL/HTTPS.
  • The page must have a service worker with a fetch handler.
  • User engagement requirements must be met (interaction with the domain for at least 30 seconds).
  • A manifest must be present.
    • 192px and 512px icons must be included.
    • Application must have a short name and long name.
    • The display mode must be specified.
    • start_url must be specified.


If all of these requirements are met Chrome will trigger a beforeinstallprompt event for the web page. Once this event is triggered your application can present the user with an install prompt.  Depending on the Chrome version your application may be able to suppress this prompt and display it to the user later (allowing you to decide where in the interaction flow that the prompt shows up) or your app might not be allowed to suppress it.

I’ll make a minimilastic application that satisfies the requirements for being a PWA.  The application that I’ll make will calculate sidereal time. Sidereal time is a time tracking system used by astronomers and is always expressed in 24 hour format. The usual system of tracking time was formed around trying to map the time of the day to the position of the sun (solar time, though it is far less than perfect). Sidereal time is based on the position of the stars relative to the observer. I will not talk much about the algorithm behind this calculation much here. I talked about calculating sidereal time in an application I had made for the now defunct Windows Phone 7; while that OS is no more the description I gave on how sidereal time works is still applicable.

Using SVG I’ve made a simple 24 hour clock face. The clock face is really there for aesthetics. Chances are if you try to read the hands of the clock the hour hand will cause confusion since it’s position on a 24 hour clock will not meet expectations that have been formed from being able to read a 12 hour clock.  The digital readout is the part that will actually give the information of interest. Every second the time is updated and the hands animate to their new position. There’s also a gear icon for opening the settings interface.


Satisfying the SSL/HTTP Requirement

A lot of the necessary features are only available if your application is being served over SSL. If you don’t see HTTPS in the address bar then these features simply will not work. To satisfy this requirement for now I’m using Google Firebase and the temporary URL that it has assigned to me. I don’t plan on keeping this URL forever, but at the time of this post you can play with the application over at https://siderealtimepiece.firebaseapp.com.

Satisfying Manifest Resources Requirements

The manifest for my application is in the root directory of the application. It is a JSON formatted file with information on where the program icons can be found, the starting URL, and the name of the application as it should appear on the user’s machine.

    "short_name": "Sidereal",
    "name": "Sidereal Time Piece",
    "icons": [
        "src": "./images/sidereal192.png",
        "type": "image/png",
        "sizes": "192x192"
        "src": "./images/sidereal512.png",
        "type": "image/png",
        "sizes": "512x512"
    "start_url": "index.html?pwa=true",
    "background_color": "#000080",
    "display": "standalone",
    "scope": "./",
    "theme_color": "#FFFFFF"

The Service Worker

To satisfy the service worker requirement there’s a JavaScript file in the root of this application’s files named sw.js. The service worker works in the background behind the page. For this application we only want the service worker to do two things; respond to an install event by caching the required files locally and serve up those files when needed. The list of the files that are to be cached are in an array named urlsToCache. When the service worker response to the install event it will pass this list of URLs to a call of the addAll method on the cache object. The cache object will then download the resources at these URLs and save them locally where we can use them offline.

var CACHE_NAME = 'siderealclock-cache';
var urlsToCache = [

self.addEventListener('install', function(event) {
  // Perform install steps
      .then(function(cache) {
        console.log('Opened cache');
        return cache.addAll(urlsToCache);

For the fetch event I’m using code from a Google recommendation. This handler will serve the contents from the cache when there is a cache hit and also add new files to the cache when a request is made for a file that isn’t already there.

self.addEventListener('fetch', function(event) {
        .then(function(response) {
          // Cache hit - return response
          if (response) {
            return response;
          // IMPORTANT: Clone the request. A request is a stream and
          // can only be consumed once. Since we are consuming this
          // once by cache and once by the browser for fetch, we need
          // to clone the response.
          var fetchRequest = event.request.clone();
          return fetch(fetchRequest).then(
            function(response) {
              // Check if we received a valid response
              if(!response || response.status !== 200 || response.type !== 'basic') {
                return response;
              // IMPORTANT: Clone the response. A response is a stream
              // and because we want the browser to consume the response
              // as well as the cache consuming the response, we need
              // to clone it so we have two streams.
              var responseToCache = response.clone();
                .then(function(cache) {
                  cache.put(event.request, responseToCache);
              return response;

This file must be registered as the service worker for it to be able to do anything. In one of the JavaScript files loaded by the page I check the navigator object to ensure there is a serviceWorker member (if there isn’t then the browser in which the code is running currently doesn’t support service workers). If it is there then the service worker can be registered with navigator.serviceWorker.register(path_to_service_worker).

if('serviceWorker' in navigator) {
             .then(function() { console.log("Service Worker Registered"); });

Handling the Install Prompt

If your code is running on a Chrome implementation that supports it you can defer the presentation of the installation prompt. In my case I’ve decided to defer it and make a button available in the settings UI. The variable installPrompt will hold the reference to the event object that when activated will present the user with the Chrome install UI. When the event is raised the variable is populated with the event object and the install button within my settings UI is made visible.

var installPrompt;

function beforeInstall(e) { 
    installPrompt = e;

window.addEventListener('beforeinstallprompt', beforeInstall);
    $('.installButton').on('click', function(){
          .then((choiceResult) => {
            installPrompt = null;

Testing the application on Chrome on Ubuntu Linux when I select my install button the Chrome install prompt shows.

Chrome Desktop Install Prompt
The Install Prompt that shows on Google Chrome on a desktop

Program Launchers on the Desktop

On the desktop once installed the icon for the PWA shows up in the computer’s program launcher.  It also shows up in the Chrome app list. When launched since this application was made to run in standalone mode the application runs in it’s own window with the OS appropriate buttons for going full screen, minimizing, and closing the window. My test application uses location services to acquire the longitude at which the sidereal time is being calculated. When run in a regular browser window I’m prompted each time I visit the page to give permission for location information. This gets a little annoying after a while. When the application is running in stand alone mode the application’s border shows an icon indicating that the location is being detected. Clicking on the icon gives the user the ability to change the location permissions for the application.

Samsung Internet Compatibility

Samsung Internet, the default browser for a long period on many Samsung phones, also supports PWAs. (Samsung Internet can also be installed on non-Samsung phones). Samsung Internet is a Chromium based browser and Samsung is one of the contributors to the Chromium project.  It may come as no surprise that no code changes are necessary for this application to work on  The UI it presents for installing PWAs is different than what Chrome presents. When Samsung Internet detects that a page can be installed as a PWA an icon is shown in the address bar that resembles a house with a plus in the center. Selecting it will add the icon to the home screen. The icon shows with a smaller image of the Samsung Internet icon indicating that it is a PWA.  The beforeinstallprompt event will never be triggered. Since the presentation of the custom install button was driven by this event it simply will not show.


Adding iOS Compatibility

If you saw the original iPhone announcement back in 2007 Steve Jobs had announced that making apps for the iPhone could be done with HTML; at the time there was no SDK available to developers and if they wanted to target the iPhone they were making a web app that had an icon on the home screen. From 2007 to 2018 Apple didn’t do much to advance the platform. It wasn’t until March 2018 that Apple made significant updates to their support to HTML based applications. Apple added support for web manifest, and services workers, web assembly, and other features.

There’s not 100% parity between iOS and Android for available features in PWA. On iOS storage is limited to 50MB per app. On Android the application can request more storage. Android BWAs also have access to Bluetooth features, speech recognition, background sync, and other features. For my sample application none of these mentioned differences matter. While the Android implementations have UI notifications that let the user know that the app can be installed on iOS there’s no visual notification. To install the application the user must select the share option and add the page to their home screen.

Safari ignores most of the attributes of the manifest. It also doesn’t save state if the user leaves the application. So the developers must make their own implementation to save state as the user jumps in and out of the application. If you want a custom icon to show in Safari for your application Apple has a document on specifying the icon using the link tag. An icon can be specified like the following.

    <link rel="apple-touch-icon"  href="./images/icons/apple-icon-57x57.png">

If you want to specify multiple icons and allow the phone to select the most appropriate one for the user’s resolution add a sizes attribute to the tag.

   <link rel="apple-touch-icon" sizes="57x57" href="./images/icons/apple-icon-57x57.png">
    <link rel="apple-touch-icon" sizes="60x60" href="./images/icons/apple-icon-60x60.png">
    <link rel="apple-touch-icon" sizes="72x72" href="./images/icons/apple-icon-72x72.png">

My clock icon for the program shows up in the iPhone favourites list as the following.


Offline Functionality

This application doesn’t need the internet for any functionality. It’s only inputs are the current local time and the user’s longitude. With the lack of need for any network resources and the service worker caching the required files for the application it will work just fine offline after it has been installed. If you make an application that requires network access you will want to give some thought to what to do when there is no data connection. Even if the application can’t do anything without a connection it would be better to show a friendly message than to just let the application not work.

An Alternative to the App Store

PWAs longtime might turn out to be a good alternative to app stores for some types of applications. Whether or not it is a good fit for the needs that you have will depend on the functionality that your applications require and what is available on the devices that you need to target. Apple appears to be behind on this front at the moment. But I hope that the attention that they’ve put on the platform this year to be indicative of future efforts. I’m personally am interested in what could be done when PWAs and WebAssembly are combined together. These are topics to which I hope to give a good bit of attention over the following months.