A sidereal clock

Here's a challenge for you. Google "sidereal clock" and see if you can find one to buy. What's that you say? You found one for $250? Okay, but hold on there. If you Google "digital clocks" I think you'll find quite an assortment of affordable clocks, say, in the $10 to $25 price range. That's what I mean for my challenge: See if you can find an affordable sidereal clock to purchase.

And I'm not talking about an app for your smartphone. Sure, you can certainly download a free "sidereal clock" app. No, I'm talking about a sidereal clock to place on your desk or hang on your wall. 

What is a sidereal clock? Permit me to explain.

All of our time keeping for daily activities is essentially solar time. At high noon, the sun hangs on our local meridians. It takes one day, or 24 hours for the sun to return to that spot since the previous day. But it's a somewhat complicated "return." The earth's rotation in roughly 24 hours is enough to make the sun appear to rise every morning like clockwork. But you have to remember that in the 24-hour period while the earth was completing a single rotation, it was also moving along in its orbit around the sun at something like 67,000 miles per hour. That amounts to a significant distance traveled by the earth in one day! In fact, the earth has to rotate a few minutes longer in order for the sun to return to the same spot in our sky. 

It's a moot experiment nowadays. There's nothing to prove, because it was already done many hundreds of years ago. But if you want to get some hands-on practice at this sort of thing, try it with your own equipment. Make a pinhole in a piece of aluminum foil and frame it so that it can be put firmly in place to cast a tiny solar disk on your back porch. You might have tried this technique to view the sun during a recent solar eclipse. Mark the position of the solar disk with a piece of painter's tape on your porch at a convenient time of day. Then, on the next day at about the same time, wait for the solar disk to return to the place that you marked. Check your watch to see how long it took. 

Okay. Got that result? It should be almost exactly 24 hours. 

Now try this: You can use a low-powered telescope or a cardboard tube from a used-up roll of paper towels. After dark on a clear night, position your tube, with or without optics, such that it's aimed at a bright star. Note the time that you see the star centered in the tube and then leave your telescope or cardboard tube there until the next night (don't bump the tripod in the meantime). Be sure to get out there about 10 minutes prior to 24 hours, so that you can be prepared to mark the return of the star to the same spot. You can expect somewhere around 23 hours and 56 minutes for the star to arrive at the same spot you marked yesterday. 

Why did the star return 4 minutes earlier than the sun? Because of the enormous distance to the stars, compared to the distance to our nearby sun. The diameter of the earth's orbit around the sun isn't enough of a baseline to triangulate the distance to most of the stars. So, one day's orbital journey made by the earth isn't going to have any impact on delaying the return of a star's position in your telescope field of view. You can assume that the time it takes for a star to return to the same spot in the sky is the same amount of time it takes for the earth to rotate once on its axis.

By measuring the amount of time it takes for the sun to return the same spot in the sky (24 hours) you will have demonstrated "solar time." And by measuring the time it takes for a star to return to the same spot in the sky (23 hours 56 minutes), you will have demonstrated "star time" or "sidereal time." By the way, if you want to sound smart, make sure you pronounce it correctly with four syllables: "sigh-DEER-ee-el" time!

Now that you have an idea what sidereal time means, how do we use it? I'm glad you asked!

Like solar time, where we use the position of the sun on our local meridian to mark the time of noon, we can use the position of a star or other celestial object on our local meridian to mark sidereal time. But this gets a little complicated because of the earth's proximity to the sun. The sun's brilliance is a distraction, an interference, and an interruption to our understanding of the earth's rotational period. 

Only in the darkness of a clear night can we see our earth's orientation and place in the galaxy. As the earth spins, new stars appear above the eastern horizon and the stars disappear below the western horizon. If we look to the north, we can see that Polaris (the North Star) doesn't change its position throughout the entire night, so we understand that the earth's spin axis is tilted and points in the direction of Polaris. The altitude of Polaris in degrees above the northern horizon matches our latitude. If we look to the south, the stars pass over the earth in giant arcs and so we must understand that our view of the South Celestial Pole is blocked by the large globe we're standing on. But before we can witness an entire rotation of the earth, that darned sun rises again, and all of our reference stars fade away! The sun commands our attention and becomes our only point of reference throughout the day. So, we can use the sun to mark our daylight hours, and the stars to mark our nighttime hours. Yet there is a subtle difference between these two reference systems, that we can discover only by studying them separately.

Imagine for a minute that the sun didn't exist. We, of course, would cease to exist without the sun, but forget about that for a minute, too! Also imagine for a minute that there are no clouds in our atmosphere, so that we have no hinderance to seeing the stars. In this scenario, the stars will be our only reference points to mark time. And there would be no daytime. Only nighttime 24/7. But the earth's rotation would still cause the stars to rise and set. It would appear to us as though the stars are fixed on a giant sphere overhead that is slowly spinning around us. 

Now ask yourself this question: "How would we mark time?" There wouldn't be a brilliant sun that blots out all of the other lights in the heavens. Instead, there would be thousands of stars to use as reference points. Which one of them should we use to mark the time of day?

When we travel around the earth, what do we use for reference points in our navigation? Especially at sea, where there are no landmarks. Cartographers overlaid an imaginary grid around the world, and they divided the 360 degrees of the globe of the earth by 24 hours in a day, in sync with solar time. This gave us lines of longitude spaced 15 degrees apart. Lines of latitude were established by assigning 90 degrees from the poles to the equator. Positive degrees for the northern hemisphere and negative degrees for the southern hemisphere. The sun and stars pass overhead at the rate of 15 degrees per hour. With a properly set clock and an accurate measurement of the sun's (or star's) altitude and azimuth, navigators can calculate their latitude and longitude on the earth. 

Likewise, astronomers used a grid reference system to map the sky. Just as you'd locate a city on earth by its longitude and latitude, so are the stars located on the celestial sphere, using what astronomers have called Right Ascension (longitude) and declination (latitude). The sky has a North Celestial Pole and a South Celestial Pole, as well as a Celestial Equator. Declinations north of the Celestial Equator are positive, and declinations south of the Celestial Equator are negative. And just as cartographers needed a starting point (Greenwich, England) for longitude, astronomers had to choose a starting point for Right Ascension. The most obvious choice was to use the position of the sun, where it crosses lines of declination in the celestial grid. These positions became the reference points for sidereal time.

Zero hours of Right Ascension (R.A.) was assigned to the spot where the sun crosses the Celestial Equator on the Spring Equinox, about March 22nd. That position puts the sun at 0 hours of R.A. and 0 degrees of declination, an appropriate starting point. For the next couple of months, the sun continues its climb in declination toward the Summer Solstice on June 22nd, where it reaches 6 hours (one quarter of its orbit) of R.A. and +23-1/2 degrees of declination, somewhere in Gemini, its highest point in the sky. After the Summer Solstice, the sun starts its downward slope toward the Autumnal Equinox, on September 22nd. Once again, its declination is 0 degrees, on the Celestial Equator, but it's now at 12 hours of R.A, in the constellation Virgo. From there, the sun continues southward as it reaches its lowest point in the sky at 18 hours of R.A., in Sagittarius, and -23-1/2 degrees of declination. This is the Winter Solstice, on December 22nd. Finally, the sun begins its climb northward to the Spring Equinox again. 

The sun's path (ecliptic) through the constellations.
Note that hours of R.A. increase from right to left.
(Graphic from Wikipedia)

Now, when the sun reaches the equinox in September, that is significant to our purposes here, because it means that, at 12 noon solar time, the sun also happens to be positioned at 12 hours of R.A. In other words, during a hot minute in September, your wristwatch reads the exact same time as your sidereal clock! Six months (half an orbit) later, your wristwatch will be 12 hours behind sidereal time, and then the following September, solar time catches up with sidereal time, so that they are equal again (briefly).

Get the picture? Yeah, sidereal time gains approximately 4 minutes per day (times 30 equals 120 minutes), or 2 hours per month over solar time. Thus, you can easily guesstimate sidereal time by adding 2 hours per month since last September to the time on your wristwatch. 

But who wants to stop and do the math every time they want to check the sidereal time? Wouldn't it be nicer to have a wall clock that you can look at and instantly know the sidereal time? All it requires is to have a clock that runs faster by 4 minutes per day over a normal clock.

What's that you say? Why do we need to know the sidereal time? 

Once again, I'm glad you asked! Well, only astronomers (amateur and professional) are likely to give two hoots about sidereal time. But here's an insight into what they do with it. 

These days there are planetarium apps on our PCs and smartphones that can show us what's up in the sky, day or night. So, even if you're familiar with the stars and constellations, you may have never felt the need to learn the usefulness of sidereal time. Your PC or smartphone has shown you the way. But what if you're out somewhere, like on Gilligan's Island, and there's no power for your PC or cell service for your iPhone? Without these powerful tools, how do you know which constellations will be overhead when the sun goes down today? All kidding aside about being on Gilligan's Island, let's imagine that you just bought your first telescope, and you'd like to see how well it performs on the Orion Nebula. Will the great nebula grace your skies tonight? How can you know?

It's not that difficult, really! Have you ever noticed those grid lines and numbers on your star charts? Yeah, these are the Right Ascension and Declination I mentioned a few paragraphs ago. The hours of Right Ascension are how we can tell sidereal time. As usual, the astronomers were way ahead of the rest of us! The celestial reference grid they made hundreds of years ago turns your local meridian into the hour and minute hands of a sidereal clock! 

It works like this: If you don't know your current, local sidereal time, you can simply go out and look up at the stars. Find one that you can identify that's very near or preferably exactly on the meridian. Then look up that star's R.A. on Wikipedia. The hours and minutes of its R.A. will be close enough to your local sidereal time, so that you can use it to set your sidereal clock. Once that task is complete, you can compare the R.A. of any celestial target to your sidereal clock and know how many hours east or west of the meridian your target is. The Orion Nebula, for example, is at 5h 35m of R.A., which can be formatted as a time: 05:35 Local Sidereal Time. That is, when your sidereal clock reads 05:35, the Orion Nebula will be on your local meridian. If instead, the time on your sidereal clock is showing, say, 02:00, then you know that the Orion Nebula will be crossing the meridian in 3 hours and 35 minutes, the difference between 02:00 and 05:35.

Because a sidereal clock gains only 4 minutes per day, you can still use it with your local time in the daylight hours to plan tonight's observations. Let's say that it's 11:30 in the morning, and you have to work the evening shift. You won't get home till after midnight, but if it's clear, you might want to take your telescope outside for an hour or two and casually hunt down some deep sky objects. Your question will be, "What's up in the sky at 1 o'clock in the morning?" Count the hours between 11:30 AM and 1 AM, which is 13-1/2 hours. Take a glance at your sidereal clock. Let's say it shows 21:35. Add 13:30 to your sidereal time, which will make it 11:05 sidereal time when it's 1 AM local time. Now check the chart key on the inside back cover of your Pocket Sky Atlas, and look at the strip between 9h and 12h of R.A. You're in luck! Some very good deep-sky targets are crossing the meridian at 11 o'clock sidereal time! And now you have a few hours before work to plan your observations in advance, so that your time under the stars later can go a little more efficiently.  

From this example, we could say that an astronomer needs a sidereal clock in much the same way the average person needs a regular clock. If you have a meeting scheduled for 2:45 in the afternoon, don't you need a clock so that you'll know when you should start making your way to the meeting? If you're an astronomer and you're planning to observe a certain object, don't you want to know when that object will be crossing the meridian? 

When you've been observing the night sky as long as I have, you won't even need to refer to your star charts. I have memorized the sky well enough to know what's up after a quick glance at my sidereal clock. That's the reason why I want a sidereal clock. So that I can just look at it from across the room and know which stars are crossing the meridian. Day or night, clear sky or cloudy, I feel comforted somehow with the knowledge of which stars and constellations are overhead. It's a weird obsession that I can't really explain, other than to blame it on my love of the stars and the night sky. But I don't want to have to count the months since last September or look around for my cell phone and swipe through the home screens until I find the correct app. And I don't want to have to wake up my PC and launch a program. I just want a sidereal clock that's always on, always there, displaying the current sidereal time, just like I want a normal clock on the wall that instantly tells me what time it is. Is it so much to ask? 

But due to lack of demand, stemming from the lack of knowledge about sidereal time, sidereal clocks are unobtainable today. I find that very frustrating and difficult to believe. I mean, on the one hand, it makes perfect sense. These days, our computers and smartphones do everything for us. If an app for displaying sidereal time on your device can be downloaded instantly and for free, who's going to want to pay for a standalone sidereal clock? I mean, you could also ask, "Who's going to buy a set of encyclopedias for their bookcase, when there's Wikipedia?" 

Oh, and (spoiler alert!) you can essentially get the same information about sidereal time from a planisphere, by dialing in the date and time of your planned observations. So again, there isn't exactly a dire need for a sidereal clock! It's just that I don't want to have to work for it. I don't want to have to spin a dial on a planisphere, count months on my fingers, or open an app on my smartphone. I just want a dedicated sidereal time display. Why can't I order one from Amazon?

Back in the early 1980's, before there were PC's and smartphones, you could buy a sidereal clock. Willmann-Bell offered one for $79. Plenty of astronomers (and professional observatories) wanted or needed to know sidereal time at a glance. So, there was somewhat of a market for sidereal clocks. 

Analog sidereal clocks were tricky to make in those days. You had to modify the quartz crystal to get the frequency correct. But there were some clock makers who knew how to do it, and we astronomers could buy sidereal clocks from them. 

There was one such sidereal clock manufacturer in Central Pennsylvania (see Sky & Telescope, July 1984, p. 54), near where I lived at the time. My father, God rest his soul, somehow knew the owner of the company and arranged for me to have a get-together with him. Mind you, my dad did this for me only because he knew that I was interested in astronomy, not because I had expressed an interest in sidereal clocks. 

I can hardly remember our meeting, some 40 years ago. I went over to the guy's house for dinner. While we ate, he asked about my interest in astronomy. I had barely been at it for a couple of years, but I shared with him my newfound love of telescopes and all things astronomical. How I had been observing the moon and planets and some of the Messier objects. I had also begun dabbling in astrophotography. He then shared with me his interest in electronics and computers, and that he had recently started a company that made sidereal clocks for astronomers.

Looking back on it now, I see a glaring disconnect between him and me on that night and I'd love to go back in time, knowing what I know now about sidereal time! Perhaps we were afraid to reveal our respective ignorance about the topic. I was a novice backyard astronomer, incapable of understanding what anybody would do with a sidereal clock. And he was a computer and electronics nerd who was seemingly incapable of explaining it to me. Could it be that he was only interested in the engineering challenges of designing a clock to run at the sidereal rate, and maybe he also didn't have a clue how astronomers actually put them to use? I noted the absence of a telescope in his house, and he didn't talk about astronomy or stargazing as being either a hobby or passion. 

 After dinner, he gave me a tour of his workshop where he assembled the clocks. It was all very impressive, but I had no knowledge of electronics or computers! I could hardly follow his description of how a sidereal clock works. As we wrapped up our conversation, he handed me a beautiful sidereal clock in a walnut case and told me I could keep it! It had a built-in, self-charging battery in case of a power fail. And it was already set to our local sidereal time. When I got home, all I had to do was plug it in, and it would read the correct sidereal time. Cool!

A photo of my clock circa 1986

Well, I got home and plugged it in, but I had no clue what it was telling me! I didn't know what I had. The clock worked for several years and then suddenly died. By that time, I had moved to Arizona and didn't feel like reaching out to the guy, whom I didn't know very well, to discuss the repair of his gift that I still didn't understand! And now it would involve long-distance phone calls and shipping costs that I couldn't afford, either. I let it go and eventually discarded the non-working clock. 

It would be many years later before I understood sidereal time and had the genuine need for such a clock. I regretted that I had thrown mine away. I would gladly pay the price for its repair now! Meantime, I had gotten a degree in computer programming, and I wrote a C++ program that could take a snapshot of the computer clock and convert the time to local sidereal time. I used the algorithms in the book Practical Astronomy with Your Calculator, by Peter Duffett-Smith. That was a fun little project! But checking the local sidereal time now meant that I had to use my PC, open an MS-DOS window, and type "LST" on the command line. What I really wanted was that old walnut-cased digital sidereal clock, so that I could look over at it any time of day or night and see the sidereal time. 

The times had changed, though, by 2020, when I finally got around to wanting a replacement for that old walnut-cased dinosaur. Should be easy as one, two, three with Google, right? So, I searched sidereal clocks, thinking that there would be a plethora of them available at a reasonable cost. Boy, was I surprised when I couldn't find a single one! Google did help me find the obituary of the gentleman who gave me the only sidereal clock I've ever owned. 

And Google pointed the way to another guy who has a web site about his sidereal clocks, based on one that he had made for Mt. Wilson Observatory, when they, too, couldn't find a replacement for their old broken one. I reached out to him, and he said he had stopped making them. There just wasn't enough interest. For a while, the materials were relatively cheap, and he had everything in stock to make a few clocks each year. But then his suppliers wanted him to start buying their parts in quantities of a thousand or ten thousand, or else they couldn't accept his orders. So, he literally had to quit making them.

Then I found a company that offers a kit so that you can make your own LED sidereal clock by following their schematic. You buy a circuit board from them, along with all the electronic components needed, and you solder it up yourself. And then, of course, you have to build your own case. I was even willing to give that a try. But their web site looks pretty old, like from the 90's, and I am doubtful that they are still in business. I sent a message to their contact address and never heard back from them. 

Wow. Really? Here we are in the 21st century, with smartphones, Go-To telescopes, self-driving cars, and AI, but we can't buy a freakin' sidereal clock? How is that possible? Seems like such a simple thing. Surely somebody would sell a simple digital clock, with sidereal time as an option. If you can make a phone app to display sidereal time, then why can't any digital clock today be programmed to optionally display sidereal time at the press of a button or flick of a switch?

I gave up looking for sidereal clocks, but a suggestion from someone on CloudyNights (I think that's where I read it) made a lot of sense: Why not just buy a regular clock and add 4 minutes to it at the end of every day? 

Haha! It's genius! At first, I hated the idea! I wanted a clock that would run 4 minutes fast so that I wouldn't have to remember to set it ahead by 4 minutes every day. But after further thought, I had to admit that there's no better solution in our times, unless we're amateur clock makers (and I'm not). Besides, if I forget to add 4 minutes to the clock every day, I can at some point just use my computer program (or the stars) to reset it.

The only catch to using a normal clock as a sidereal clock is that sidereal time has to be displayed in a 24-hour format. So, I went with a digital clock from Amazon, made by a company called PEAKEEP (pictured above). It can be set for 24-hour display, and Daylight Savings Time can be turned off (I live in Arizona, where we don't observe Daylight Savings Time). It's in the $20 price range. The numbers are large enough to be readable from across the room. And as long as you don't set an alarm, the display is uncluttered. Just the time. Adding a few minutes per day is simple with the press of a couple buttons. I use my LST computer program to set it to the correct sidereal time, but then I add about 12 minutes so that I don't have to reset it again for a few days. That's accurate enough for my purposes. The only drawback to the PEAKEEP clock is that it goes through batteries like crazy! So, I bought one of those plug-in power adapter cords that has dummy batteries to insert into the clock's battery compartment.  

Would I prefer to have a real sidereal clock? Most definitely, yes! If you ever find them being offered for sale again at a reasonable price, let me know!