Function

The main function of this app is to help you calculate your most likely position from a series of star sightings. The resulting position is given in latitude and longitude, so you can find out approximately where you are. For users familiar with the line of position (LOP) intercept method, the app also plots the intercept lines and LOP lines. Note that StarStruck’s algorithm for determining the most likely position is a little more computationally involved than the simple LOP method, so the result should be somewhat more accurate.

To help you identify visible stars for sight recording, a star map is available if you need it. A star map displays each star’s altitude and its azimuth from true north. The large circle around the stars represents the lowest point in the sky, which we call the horizon; the center of the circle represents the highest point in the sky, directly overhead, which we call the zenith. Learning to read a star map is well worth it — it really helps you identify stars and get familiar with the night sky. It’s a tried-and-true method, just as widely used today as it was by generations of navigators before us.

Usage

If you’re already familiar with sextant navigation, continue reading below. Otherwise, you may want to read the background section on celestial navigation first to familiarize yourself with the concept.

To determine your position, you’ll need to take a series of star sightings — I’d recommend at least three different stars, though more is better — and record the following information with the help of a sextant:

1. GMT time of the observation
2. Star name: SUN, MOON, Polaris, Sirius, Altair, etc.
3. Altitude of the star, in degrees and minutes
4. Other info: height of eye above sea level, sextant index correction, and lower or upper limb (required only for the Sun and Moon)

The app lets you enter all of this information, then calculates your most likely position for you.

Integrated Sextant Usage

Since most of us don’t own a sextant, I’ve made use of the sensors available on most smartphones to measure a star’s altitude just by pointing the device at it. Sensor accuracy is within about 1 degree, so it’s not as precise as a good sextant, but if you take several star sightings, the errors will average out and you’ll still get a good fix on your position.

To use the integrated sensor for altitude measurement, tap the Configure button and check “Use Device as Sextant.” For aiming, I place the phone on top of a straw and look through the straw to find the star. Once the star is in view, I press either the track ball or the volume control buttons to record its altitude.

When you start the app for the first time, sensor-based altitude measurement is on by default. You can switch to manual sextant input at any time using the Configure button.

Artificial Horizon Usage

Artificial horizons were used by sailors when fog obscured the natural horizon. They’re also a great tool for those of us who aren’t lucky enough to live by the sea and see a natural horizon, letting us practice celestial navigation anyway. If you don’t have an artificial horizon handy, a tray of motor oil works just as well. Let’s use a sun sighting as an example — here are the steps:

1. Bring the sextant to your eye and move the index arm until you see two suns — one reflected on the liquid, and a doubly reflected image in the mirrors.
2. Continue moving the index arm until the two suns line up. The angle you read from the sextant is now exactly twice the sun’s true altitude.

When you enter this angle into StarStruck Navigation, it will automatically halve it for you. Height of eye is set to zero, so no dip correction is applied.

Control Buttons

Here are the main control buttons.

Configure

Set your user preferences. Use this button to enable the device as an altitude-measuring instrument — otherwise, you’ll be asked to enter sextant measurements manually. It’s also how you clear the star sights stored in memory and start fresh for a new position estimate.

Record

Start recording a star sight. The same function can be triggered by pressing the volume control buttons, or the track ball if your phone has one. Every recorded star sight is saved in memory. You’ll need at least two star sights before you can get a fix on your location — the more you record, the more accurate the calculation.

Locate

Calculate your most likely position based on two or more recorded star sights. Use this button after you’ve recorded at least two sightings.

Star Map

Show a map of all stars visible from your most likely location, or from a manually entered location. You can change the star map’s location via the star map menu (Starmap screen > Menu > Set Location). By default, the star map uses your phone’s current time, but you can change it to a different time (Starmap screen > Menu > Set Time). To see a full listing of each star’s altitude, azimuth, GHA, and declination, use the star map menu (Starmap screen > Menu > Display Star Data).

Default Settings

User Preferences:

Advanced Usage

There are a number of features available to support more advanced users. These are:

1. LOP Editing

LOP editing lets you review the line-of-position (LOP) data captured from every star sight you’ve recorded. You can delete an LOP, or edit it to account for the ship’s movement between sightings. Access it via Menu > LOP. To adjust an LOP for ship movement, use the ship-movement input dialog and the Record button; to remove one, use the Delete button.

2. Trip Editing

Trip editing is accessible via Menu > TRIP. It lets you organize your star sight records into groups within a trip — you can have many trips, and each trip can contain many groups of star sightings. A group typically corresponds to one geographical location whose coordinates you’re trying to determine.

A trip may contain one or more groups of star sightings. Trip editing lets you add or remove trips, and add or remove groups within a trip. You can set the active trip and group using the Config button.

“Trip” and “group” can be a little confusing, so here’s an example. Say you’re sailing from New York to Sydney, Australia. At the start of the voyage, you’d go to Menu > Edit Trip and create a trip called NewYorkToSydney. As you travel and want to determine your ship’s location at different points along the way — say, location #1 through location #10 — you’d take two or more star sightings at each location. Each set of sightings for one location is what the app calls a group. In this example, you’d create 10 groups to hold the sightings for your 10 locations. In summary: one trip, NewYorkToSydney, containing 10 groups. Organizing your sightings this way lets you review them later.

If you have no interest in saving your star sightings for later, there’s no need to bother with trips and groups at all.

By default, StarStruck saves every sighting to a group named “defaultGroup” under a trip named “defaultTrip.” If you’re not planning a specific trip and just want to record sightings as you go, the default is fine — no need to create anything else.

3. Offline Maps

The app supports offline maps, which is helpful when the Internet isn’t available. Since an offline map of the entire world would be huge, the app ships with only a very low-resolution world map to keep its size small. If your device has enough external storage, you can download higher-resolution maps yourself to support offline navigation.

To download offline maps, first install Mobile Atlas Creator (MOBAC) on your PC — a free program available at mobac.sourceforge.net. It lets you define a map area, choose a map source (pick OpenStreetMap Mapnik), and choose an atlas format (pick osmdroid ZIP). Download the resulting ZIP file and place it on your SD card under /sdcard/osmdroid. After that, StarStruck can display the map even without an Internet connection. For faster map loading, unzip the file directly into /sdcard/osmdroid so StarStruck doesn’t have to unzip it every time a map is needed.

4. Meridian Passage, Rise, and Set Times

Accessible via Menu > Meridian. Users often want to know what time the Sun or Moon passes overhead (meridian passage), or when it rises or sets at their location. This feature provides a quick lookup of that information.

5. Magnetic Variation

Accessible via Menu > GyroError. Knowing your compass’s magnetic variation lets you correct its reading to find true north. Since the magnetic variation is simply the difference between true north and magnetic north, you just add it to your compass reading to get true north.

Accuracy

The accuracy of the computed position depends on your measuring equipment. According to Wikipedia:

“Professional sextants use a click-stop degree measure and a worm adjustment that reads to a ‘minute of arc,’ 1/60 of a degree. Most sextants also include a vernier on the worm dial that reads to 0.2 minute. Since 1 minute of error is about a nautical mile, the best possible accuracy of celestial navigation is about 0.1 nautical miles (200 m). At sea, results within several nautical miles, well within visual range, are acceptable. A highly skilled and experienced navigator can determine position to an accuracy of about 0.25 nautical miles (460 m).”

Celestial Navigation Background

Rather than writing a complex mathematical tutorial on celestial navigation and scaring off 90% of readers, let me start simply: celestial navigation is another way of determining your position on Earth, using the stars as your lighthouses.

I’m using the term “star” loosely here to include the Sun, the Moon, and the planets as well.

Most of us are familiar with lighthouses as a way to help sailors determine their position and find their way to a safe port. By tracking lighthouses along your route on a map and following them, you should be able to reach your destination without much trouble.

Now suppose someone gave you a map showing exactly which stars are visible in the sky from any location on Earth. Surely you’d be able to look up, compare the visible stars to your map, and figure out your approximate location — wouldn’t you?

I’m glad you answered “yes,” because that means you’ve grasped the fundamental principle of celestial navigation. If you answered “no,” go have a coffee and think about it again — I’m not taking no for an answer here :)

In the past, that map of the sky was contained in huge, intimidating volumes called the Nautical Almanac — different editions had cryptic names like HO 229 or HO 246. Learning to use these books really took the fun out of celestial navigation, reducing it to laborious, error-prone work. In the old days, keeping this science a secret made a certain kind of sense — if only the captain and his first mate knew how to navigate, mutiny was unthinkable. But there’s no need for that kind of secrecy today. We want to make celestial navigation easy, so more people can learn to appreciate it — otherwise, this beautiful ancient art won’t survive.

A navigator armed with these books would use a sextant to measure the altitudes of different stars — this is called star sighting — and record the time of each sighting. Using the recorded times and altitudes, he could look up in the almanac approximately how far he was from an arbitrarily assumed position.

Having suffered through this process myself, I built this app to do away with the books and their table lookups entirely. When you use StarStruck Navigation, all the table lookups and calculations happen automatically. All you have to do is enter the sighting time, the star’s altitude, and your assumed position — the software takes care of the rest and gives you your approximate location.

For the technically inclined, I’d recommend reading “Celestial Navigation for Dummies” by onboardintelligence.com.

History of the Sextant

No discussion of celestial navigation would be complete without a look at the history of the sextant and our good old watch — two key devices without which celestial navigation simply couldn’t work. I hope you find it as fascinating as I do.

The sextant’s history is probably as old as humanity’s first attempts to navigate by the stars — which is to say, we don’t really know exactly when it all began.

What we do know is that, as early as a thousand years ago, sailors were using Polaris’s altitude to find their way back to home port. First, the sailor would record Polaris’s altitude at his home port. Then, to return after a long voyage, he needed only to sail north or south until Polaris returned to that same altitude, then turn left or right as needed and “sail down the latitude,” keeping Polaris at a constant angle.

Arab navigators knew this technique well, and they introduced two important instruments for measuring the altitude of the stars: the quadrant and the astrolabe.

In the word “astrolabe,” astro means ‘star,’ and labe roughly translates as ‘to take’ or ‘to find.’ To use it, the navigator held the instrument by the ring at the top, letting it hang in a vertical plane, then aligned it toward the object of interest. The alidade was pointed at the object, and the altitude was read off the outer degree scale. The astrolabe’s weakness was that it had to hang freely and vertically to give an accurate reading — fine on land, but a real problem on a ship in rough seas.

Similar to the astrolabe, the mariner’s quadrant — a quarter-circle of wood or brass — came into widespread use for navigation around 1450, though its use can be traced back at least to the 1200s. It’s simpler than the astrolabe, being only a quarter of its size, but shares the same weakness: it doesn’t work well on rough water.

During the 1400s, Portuguese explorers sailed south along the coast of Africa in search of a route to the Orient. As a ship nears the equator heading south, Polaris drops below the horizon — so mariners in southern seas needed a different way to find their latitude. Under orders from Prince Henry the Navigator, Portuguese astronomers had, by 1480, worked out how to determine latitude from the sun’s seasonal north–south drift, what we now call its “declination.” In simple terms, a navigator could find his latitude by measuring the sun’s altitude at its highest point — local apparent noon — with a quadrant, then applying a correction based on the sun’s declination for that date.

The next step in the evolution of celestial navigation instruments was the cross-staff, a device resembling a cross. It’s made up of two pieces: a long staff, and a shorter crosspiece called the transom (or limb), which slides along the staff so the star can be sighted over its upper edge while the horizon lines up with its bottom edge.

The navigator placed one end of the main staff against his cheek, just below the eye, and sighted the horizon along the bottom edge of the transom while sliding it along the staff until the sun (or star) lined up with the top edge. The altitude could then be read from the transom’s position on the staff’s scale, converted to an angular measurement using a lookup table.

The cross-staff’s major drawback was that the observer had to look in two directions at once — along the bottom of the transom to the horizon, and along the top to the sun or star. Not an easy feat on a rolling deck!

The backstaff, or back-quadrant, was another step in the evolution of celestial measuring instruments, designed specifically for observing the sun. Users kept the sun at their back (hence the name) and observed the shadow it cast onto a horizon vane. It was invented by the English navigator John Davis, who described it in his 1594 book Seaman’s Secrets. It’s called a quadrant because it measures up to 90 degrees — a quarter of a circle. The observer found the sun’s altitude by watching its shadow while simultaneously sighting the horizon. Relatively inexpensive, sturdy, and reliable, Davis quadrants remained popular for more than 150 years, even after far more sophisticated double-reflection instruments were invented.

One major advantage of the Davis backstaff over the cross-staff was that the navigator only had to look in one direction to take a sight — through the slit in the horizon vane, while aligning the shadow vane’s shadow with that same slit.

The major problem with back-sight instruments was that they made it difficult, if not impossible, to sight the moon, planets, or stars. So toward the end of the 1600s and into the 1700s, the more inventive instrument makers began shifting their focus to optical systems using mirrors and prisms — ones that could observe nighttime celestial bodies as well.

The critical breakthrough came independently, and almost simultaneously, from John Hadley in England and Thomas Godfrey, a Philadelphia glazier, around 1731. The core idea: use two mirrors to create a doubly reflecting instrument — the forerunner of the modern sextant.

To use a sextant, hold it vertically and point it toward the celestial body. Sight the horizon through the unsilvered part of the horizon mirror, then adjust the index arm until the image of the sun or star — reflected first by the index mirror, then by the silvered part of the horizon mirror — appears to rest right on the horizon. The altitude can then be read from the scale on the instrument’s arc.

Like the Davis quadrant, the sextant measures celestial objects relative to the horizon rather than relative to the instrument, which allows excellent precision. But unlike the backstaff, the sextant allows direct observation of stars, so it can be used at night, when a backstaff is difficult to use. For solar observations, filters allow direct observation of the sun.

A sextant doesn’t require a completely steady aim, because it measures a relative angle. For example, when a sextant is used on a moving ship, the images of both the horizon and the celestial object move around in the field of view — but their relative position stays steady. As long as the user can tell when the celestial object touches the horizon, the measurement stays accurate despite the ship’s motion.

History of Time Keeping

Seafarers could determine their latitude using a sextant to observe the sun’s position at midday, or bright stars at night. And by tossing a weighted, knotted rope off the ship and counting how many knots passed through their hands in a given time, they could get a rough estimate of speed. But it’s of limited use to know you’re traveling southeast at 7 knots, 50 degrees north of the equator, if you have no idea whether you’re closer to Newfoundland or Ireland. In other words — what longitude are you on?

The key, as everyone realized, was knowing the time both at your current location and at some fixed reference point on the planet — say, Greenwich. Because the Earth’s rotation speed is known (about 15 degrees per hour), knowing the time difference between your location and Greenwich lets you do some simple math and arrive at your longitude.

Unfortunately, building an accurate clock wasn’t easy — at least not in the 15th and 16th centuries. The pendulum clocks of the day were useless on a rocking sea; they simply weren’t accurate enough to navigate with any certainty, and this all too often led to disaster.

There being no apparent solution, mariners simply lived with this frustration. But on a foggy day in 1707, four large British warships returning from France misjudged their longitude and ran aground on the Scilly Islands, off the southwest tip of England. The ships all sank, and more than 2,000 men died. As a result, in 1714 the British government offered a huge cash prize — 20,000 pounds, equivalent to millions of pounds or dollars today — to the first person who could devise an accurate way to measure longitude at sea. To win, a solution had to resolve a ship’s position to within 30 nautical miles after a voyage to the West Indies, which meant a clock accurate to within 3 seconds a day.

For half a century, all manner of charlatans and pseudo-scientific crackpots tried to claim the prize, but none could solve the fundamental scientific problem of determining longitude at sea. During this time, John Harrison (1693–1776), a woodworker and musician from Lincolnshire, devoted his life and genius to solving the problem.

Through intuition and sheer determination, he developed a maritime chronometer accurate to within one second a day — a huge advance, one that overcame the harsh conditions of the open sea. Using a copy of Harrison’s clock, Captain James Cook mapped the Polynesian islands and the Pacific. He praised the new instrument in his logbook as “our trusty friend the watch” and “our never-failing guide.” It’s a shame that the skeptical Board of Longitude, made up of astronomers with little interest in how a clock actually worked, kept denying Harrison his prize until he was in his eighties. Still, better late than never.

Over time, clocks have improved dramatically and needs have changed, so GMT has evolved into UTC. The acronym UTC is an English–French mixture for Coordinated Universal Time. UTC is based on a definition of the second that is nearly a million times more accurate, and under the general umbrella of la Convention du Mètre, this new second is defined by a quantum resonance within a cesium atom.

It’s worth noting how much timekeeping precision has improved with today’s technology. The current record, set by the NIST-F2 clock announced by the US in 2014, keeps time accurate to within plus or minus one second every 300 million years. As astonishing as that is compared to Harrison’s chronometer, we should always pay tribute to the man who first lifted the art of navigation to the level of precision it so critically needed.

Skymap Memory Aids

To help identify stars in the night sky, people have long relied on prominent landmarks — the Big Dipper, Orion, the Southern Cross — to quickly locate nearby constellations and stars. This method is useful for beginners and experts alike, since the night sky can be complex and it’s easy to forget how everything is arranged. Hopefully this short guide can help you get a handle on it.

The Big Dipper

The Big Dipper is a prominent group of stars in the northern sky and is very readily recognizable — it looks like a bowl with an arching handle. It’s circumpolar (never setting below the horizon), so it’s visible in northern skies year-round. If you can find the Big Dipper, you have a starting point for identifying many other stars. As you can see from the diagram, numerous other stars can be found based on its position. Let’s see how to identify a few of them.

1. “The Pointers” — the two stars forming the pouring edge of the Big Dipper’s bowl (on the side away from the handle) point to Polaris, the North Star, in the constellation Ursa Minor. Polaris is a rather faint star, about five times farther away than the distance between the pointers themselves. Note 1: no matter where you are in the northern hemisphere, when you face Polaris you’re facing north. Note 2: Polaris marks north more accurately than a magnetic compass. Note 3: the angle between your horizon and Polaris equals your latitude on Earth.
2. Continue that line from the Pointers past Polaris, and at an equal distance on the opposite side of the Big Dipper, you’ll intersect Cassiopeia — a W-shaped constellation home to the stars Caph and Shedar.
3. Trace the line from the Pointers through Polaris and past Cassiopeia, and you’ll reach a large, nearly perfect square of four stars called the Great Square of Pegasus.
4. “Follow the arc to Arcturus” — follow the curve of the Big Dipper’s handle away from the bowl to the fourth-brightest star in the sky, Arcturus, in the ancient constellation Boötes.
5. “And speed on to Spica” — extend that curve another Dipper-length past Arcturus, and you’ll meet Spica, in the large zodiac constellation Virgo.

Orion

After the Big Dipper, Orion is the most distinctive group of stars in the heavens. Rigel is the brightest star in Orion, followed by Betelgeuse. The three stars in the middle of Orion form its “belt.”

1. If you extend the line of Orion’s belt about 20 degrees down and to the left, you’ll find Sirius, the brightest star in the night sky. Note: hold your fist at arm’s length and it spans roughly 10 degrees, so 20 degrees is about two fists. One degree, meanwhile, is roughly the width of your little finger at arm’s length — a handy trick for estimating stars’ relative positions when you don’t have a precise measuring instrument on hand.
2. Go the same distance (20 degrees) in the opposite direction, and Orion’s belt points to Aldebaran, a first-magnitude, yellowish-orange star. Extend that line another 15 degrees, and you’ll reach the beautiful star cluster known as the Seven Sisters — the Pleiades.
3. Draw a line from the middle of the belt through the midpoint between Betelgeuse and Bellatrix, and extend it another 45 degrees — you’ll reach the constellation Auriga, home to Capella, the second-brightest star of the winter night sky, just behind Sirius.
4. Trace a broad curve from Capella to Sirius, passing to the left of Orion, and you’ll meet three more bright stars: Castor, Pollux, and Procyon.

The Southern Cross

To find your way around the southern sky, it’s best to locate the Southern Cross first and use it as an anchor point. Though small, and less prominent than a constellation like Centaurus, it’s distinctive enough to serve as a reliable point of reference.

1. To locate the Southern Cross, trace a line through Rigil Kentaurus and Hadar, in the constellation Centaurus, toward Gacrux — the star at the top of the cross.
2. Once you’ve found the Southern Cross, you can use it to locate the South Celestial Pole — the pivot point around which all southern stars appear to orbit. Draw a straight line between Acrux and Achernar, then bisect it; the midpoint is approximately the South Celestial Pole.
3. The constellation Carina, known as the keel, lies to the lower left of the Southern Cross. It’s home to the stars Canopus and Miaplacidus.
4. The constellation Phoenix, home to the star Ankaa, can be found by extending the line from the Southern Cross through Achernar — Phoenix lies just about 5 degrees beyond it.

Keep in mind that although the stars themselves don’t move, our constantly spinning, orbiting Earth makes them appear to “move” across the sky over time — much like buildings appear to slide backward past the window of a moving car. Star maps rotate and shift higher or lower in the sky from one season to the next, but the relative positions of the constellations and stars never change. So you can always rely on a star map for guidance, even as the stars appear to drift.

The Beaufort Wind Scale

The Beaufort wind scale is a simple way for sailors to relate observed sea or land conditions to wind speed. I couldn’t leave it out — not just for its usefulness in estimating wind speed, but for its beautifully simple, almost poetic descriptions of each level. There may never have been a more succinct description of the wind than the Beaufort scale. If you’re curious, there’s a whole book about it: Defining the Wind: The Beaufort Scale and How a 19th-Century Admiral Turned Science into Poetry, in which Scott Huler tells a wonderful account of its eccentric creator, Sir Francis Beaufort, and how the scale came to be. I hope you come to see its beauty and usefulness too.

Choose Your Weapons

Although this app can use your device’s built-in sensors to measure a star’s altitude, accuracy is limited to about a degree. That’s a handy fallback when you don’t have a sextant, but if you do have one, use it instead. For anyone on a budget, here’s what I’d recommend:

1. Casio G-Shock watch (~$80): excellent, water-resistant, and shock-resistant. It doesn’t need a battery — it runs on solar power — and syncs daily to an atomic clock radio signal. It can also display GMT directly, so there’s no need to do any time conversion.
2. Davis Mark 25 sextant (~$200): an inexpensive sextant with accuracy up to 2/10 of a minute of arc. Many seasoned navigators recommend it for its accuracy and durability.

When you’re on the water, make sure all your electronics are well protected. I’d recommend a waterproof case for whatever device you use for celestial navigation. I’ve gone into the water with my phone before — not by choice — so I know it can happen to anybody. And if you’re heading out on a long voyage, bring a solar charger, just in case your battery dies and you’re stranded on a deserted island. :)

Please report any problems by email to starstrucknavigation@gmail.com — that’s far more helpful to me than a review on the app store. I’m always looking for ways to improve the app, and I read every comment carefully, working what I can into later releases. My goal has always been to make celestial navigation fun and approachable, leaving out anything cryptic or mathematically cumbersome, while still keeping it useful for advanced users.

Free Download

Click the link below to download a free copy of the app. I’ve since retired from software development, so please don’t ask me for further modifications.

Thank you for using StarStruck Navigation.

I appreciate any donation to help fund this work and keep the price affordable for everyone. Thank you.