Sextants and the History of Celestial Navigation

[@TheWizardGamez]

This channel is so underrated. And so is literally every other decent history channel.

[@rickkwitkoski1976]

@TheWizardGamez Try The History Guy. Pretty good History channel. Getting close to 2 million subs.

[@TerryClarkAccordioncrazy]

Issuing your own corrections is a superb example of the scientific method, deserving of much respect.

[@Ahnii]

and starting the videos with them, really goes to show how much he cares!

[@robertwilson8789]

As a professional merchant marine officer retired we used celestial nav several times a day regardless of nice electronic nav aids aboard ship . It is required by most shipping companies as a way to keep the skills up in case of equipment failures . Using celestial in conjunction with electronics that are pretty accurate like gps (even when fewer sats were in the air ) was a good way to compare accuracies and techniques of your observations .

[@stephenbritton9297]

By the time I was shipping, the sextant generally sat gathering dust on the bridge. We had a Kings Point (US Federal maritime academy, and many get programmed with a arrogance while there) cadet on board who challenged me (2nd mate) to see who was better over a full days run. He figured I was out of practice and could show off his superiority. We when we DR’d our plots and compared it to the GPS, we straddled the GPS fix perfectly, and had to call it a tie.

[@blarpnarp]

@@stephenbritton9297 theres no way you guys totally didnt become best friends that day

[@simonwinwood]

@@stephenbritton9297❤

[@oldmech619]

Celestial navigation is not easy if you do it by the book. If the day comes that you need it, you need to be your skills up

[@robertwilson8789]

@@oldmech619 Sure isn't easy and even the sextant you use will have little idiosyncrasies. Even though most of the ships I sailed on had sextants on the bridge , I always carried my own Plath instruments mk3 that was standard issue on Navy ships and had excellent machining and good lenses .That and my hand bearing compass .

[@RevMikeBlack]

The fact that you acknowledge previous errors and offer corrections is really impressive... enough to make me want to subscribe. Thanks.

[@Oldtimerider]

Great presentation! At least until 2003, when I stopped flying C-130’s in the Marines, very talented young ENLISTED navigators were guiding us around the globe using celestial navigation as a backup to GPS and INS.

[@rabaohong9492]

You are one of the best educators on the entire Internet. Extremely clear and precise. Every lesson is awesome.👍

[@Ahnii]

Your dedication to historical and technical detail is awesome and having your corrections at the start rather than the end really shows how much you care!

[@fredblonder7850]

Regarding the periscopic sextant, these were especially useful when fitted to the periscopes of submarines, which allowed them to obtain their position without surfacing. Really handy in time of war. Greenwich England was established as the universal prim meridian in “about” 1880. Prior to this, every major port had its own prime meridian, and ships’ charts would all be calibrated relative to their home port. As you can see, this led to massive confusion, but extra business for mapmakers. In Washington, DC, one of the public parks is Meridian Hill Park, which encompasses the old Washington DC Prime Meridian. I have been to Greenwich and was shocked to find that Harrison’s original chronometers are not only on display, but #1 - #3 are running continuously. Only H4 is run one day a year because it lacks the bearings of the earlier chronometers, and will eventually wear out. A method of celestial navigation I learned was to plot the sub-stellar point of three objects onto your chart, then use a sextant to measure their angle from the zenith and use that to set the diameter of a circle you would draw, centered on the sub-stellar point, then see where the perimeters of the three circles intersect, which gives your location. This is probably no more accurate, but lets you clearly visualize what’s going on. The “Moons of Jupiter” method of navigation may not have worked well at sea, but was fantastic on solid ground and was used by the famous astronomer/surveyors Mason and Dixon who laid-out the Mason-Dixon Line, which separates the Northern United States from the Southern States. As this is closely tied to the American Civil War, it still has political meaning to this day, with some people unaware of the history of the line opining that Mason And Dixon were racists or some such nonsense.

[@CanadianMacGyver]

Yes, I've seen the Harrison chronometers at Greenwich as well; in fact, that was my primary reason for visiting! I did not, however, know that Mason and Dixon used the Jovian Satellites Method; that is fascinating! Thanks for sharing :)

[@fredblonder7850]

@@CanadianMacGyver Pardon me if I’m telling you something you already know, but the 1869 novella “The Brick Moon” by Edward Everett Hale is the first Science Fiction story to describe an artificial Earth satellite. What many people overlook is that the Brick Moon was to be placed into a polar orbit and used to solve the Longitude Problem. In other words, the first conceived use for an artificial satellite was as a Navigational Satellite. I don’t intend to turn this into a review, but in the story the launch is botched and the Brick Moon winds up in the wrong orbit, carrying the plot off in an odd direction as well.

[@Inflorescensse]

@@CanadianMacGyverof course Jack Aubrey invented the navigational system based on Jovian satellite nutation.

[@michaelimbesi2314]

More accurately, the Mason-Dixon Line is the border between Pennsylvania and Maryland.

[@dozerboy67]

I sailed on a two masted tops’l schooner, traditionally rigged and we didn’t have any satellite navigation. We did it all by charts and compass and using the navigation aids such as light houses and beacons, but when we were out at sea the only way to determine our location was to use a sextant and a boatload of calculus! I learned a great deal about dead reckoning and using logic and reasoning to determine speed, distance traveled and location. It was an in exact science but it did get us where we wanted to go! We sailed throughout the North Sea and through the lochs in Scotland to the Irish Sea and down to the Mediterranean Sea and then through the Suez Canal and to the Red Sea. I was 15 years old and it was the time of my life! I am a proud member of the brotherhood of sailors, if you’ve been to sea you’ll understand, if not, well… it’s not something I can explain 🇺🇸🇺🇸⚓️⚓️💪👊

[@makoyoverfelt3320]

Gilles, you are a gem. This channel is very high quality, thank you for your work!

[@SirDigbyChikkinCaesar]

Finally, a concise overview of how sextants work, amazing video, thank you!

[@TheFailedPhysicist]

This is such a beautiful video! So much juicy lore and information condensed in a small package. Subscribed!

[@yebobaba]

Your videos are very interesting and informative. I have always understood the principles of navigation, but didn't know the procedure. Now I know. Thank you. Please keep them coming.

[@UniversalEngineer]

Wow! Congrats to the machinists who hand-made these beautiful brass, steel, and glass instruments! ❤❤❤

[@DennisMathias]

National Geo should help fund this channel. Just super excellent.

[@tectopic]

Why haven't I stumbled on this channel before? Really nice presentation in such a short clip. A correction though, at around 23:50, and the bubble, the bubble is there during the entire obervation: Turning the left hand wheel down, you apply pressure on a diaphragm until an air bubble pops out of it and enters a chamber of xylene-fluid. Then use the same wheel to adjust the size of the bubble until it is 1-2 mm wide (1/20", really small), and the bubble is ready for observation. Now look into the sextant, keep that bubble centred and simultaneously set the reflection of the sun/star/moon/planet in the middle of that bubble. For my Mk9A, with the averager clock, the challenge is to keep that reflection in the centre for the full two minutes. It is trickier than I assumed. Actual angular precision achieved is 1-2 arc minutes usually.

[@liquidmandotcom]

Very interesting breakdown of a very complicated subject.

[@skivvy3565]

Amazing. I cannot express how impressed I am with your channel and the quality as well and quantity

[@Adv0kaat]

Great video: well presented and explained and very thorough. Since you asked for feedback: 1) Perhaps keep the errata to a timestamped section at the end (we're here for sextants, not corrections); 2) The reflection in your glasses is distracting (and worse, disengaging because we get no eye contact). If it's your teleprompter, maybe try white text on a black background, or some sort of filter? I rarely ever comment but came across your channel recently and you've a real talent and some great content. Best of luck and thank you for your hard work!

[@CanadianMacGyver]

Thanks for the feedback! I decided to put the errata in this one because they mainly concerned the astrocompass, which tied in with the navigation theme. As for the glasses, the space I was filming in was only lit by sunlight from a facing window, and I couldn't for the life of me find a camera angle that eliminated the glare. I will be investing in photography lights/diffusers to eliminate the problem in the future. Cheers!

[@JCWren]

@@CanadianMacGyver How about a pair of contact lenses? :)

[@TimoNoko]

When you are on land expedition you cannot see horizon either. Lewis&Clark had a glass covered box with water in it. They measured the angle between sun and its reflection. And Fridtjof Nansen used mercury-filled box instead, because water would freeze solid.

[@austinhadley6086]

John Hadley is my ancestor! Excellent video

[@skivvy3565]

If anyone hasn’t seen the production documentary tv series of ‘the longitude problem’ you must go watch it!!! One if the most fascinating problems and stories in human history

[@human_shaped]

A nice addition to this video would have been the inclusion of a Bygrave slide rule, which is a cool cylindrical slide rule that simplified the calculations and tables needed for celestial navigation. It was used for marine but particularly aviation navigation. During WWII they were included in life raft kits for aircraft because they were quick and much lighter than carrying your huge HO 229/249 or other almanac.

[@heinpereboom5521]

Excellent explanation and the history is also very interesting!

[@cokdnlokd1238]

Gilles, I am a longtime celestial navigator (oceans master) you can only shoot stars, planets etc at civil twilight which occurs after nautical twilight in the morning and before nautical twilight in the evening. If you cannot see the horizon you can shoot stars unless you have some form of artificial horizon like a mercury horizon or a bubble sextant and yes in airplanes and submarines they used artificial horizons a lot of the time. I believe there are 52 navigational stars although I believe its possible to use others if you have the correct ephemeris.

[@robertwilson8789]

There are misconceptions , generally, about how celestial is typically used . The most common sights used are not even from stars , but from the sun with the easiest to tune into being LAN or local apparent noon .When the apparent hour angle is increasing you are approaching noon when the apparent hour angle begins decreasing you are afternoon . By interpolating , you can find noon pretty closely and go to your position from the logbooks . Stars were mainly used if days had been cloud covered , but night was clear

[@robertwilson8789]

Correct ! The most commonly used sighting during the daytime is the LAN sight or 'Local Apparent Noon' shot from the sun , however you use what ever is available due to atmospheric conditions . Good twilight shots will be more accurate overall .

[@Gribbo9999]

You can also use the moon but have to add further corrections for parallax as the moon is relatively quite close. On a clear full moon night it is possible to site at night sometimes, if you can see the horizon by moonlight.

[@Gribbo9999]

Polaris is another easy site if you are if you are unfortunate enough to live in the northern hemisphere. We Aussies don't have this luxury.😅

[@n5gui]

On the inverted image of the third telescope of the first sextant example, I suspect it has a higher magnification than the others, perhaps for use at night on stars or planets. I believe if you can check the eyepieces used, the upright image comes from a negative (concave) eyepiece, and the inverted image comes from a positive (convex) eyepiece. I suggest the higher magnification would have helped the sighting of tiny points of light. I hope these comments are helpful.

[@emanuelbinder4263]

Great video! Kudos from Rio de Janeiro Brazil!

[@WalkaCrookedLine]

Your discussion of aerial navigation reminds me of the analog calculation instruments my naval aviator father used in the 1960s. These odd-looking gadgets had several scales that worked on slide rule principles and were used to calculate position, fuel consumption, weight distribution and probably other purposes. In ww2 a model called the E6-B was in wide use, but I haven't been able to find one like my father used on the Internet. Unlike the E6-B, the one my father used had both circular and linear scales for different functions. I'd love to see you do a video on how these worked and how they evolved over time.

[@LincolnWallace-g4f]

sublime presentation and information, as always. though this one triggered me a little, because it left one thing disappointingly unanswered and unshown: the actual sight picture. ever since i was a child i've been itching to know what you actually see, looking through a sextant. i know how they work now .. but i'm none the wiser. T_T but my one suggestion for your collection would be a "viking sun stone" (iceland spar). it's a natural crystal with very unique optical properties, and propably the earlist optical instrument ever used in naval navigation.

[@Nupetiet]

you pull off a kind of dorky look but with badass vibes you're good

[@phillipdoorbar1615]

The simplest way of determining the altitude of the sun is to first set the sextant to zero, select appropriate shades for the index and horizon mirrors, point the telescope at the sun, then bring the image of the sun down to the horizon using the index arm. Fine adjustment can then be made with the micrometer dial. Also, a timing error of 1 second is actually 101 foot.

[@Ucceah]

sublime presentation and information, as always. though this one triggered me a little, because it left one thing disappointingly unanswered and unshown: the actual sight picture. ever since i was a child i've been itching to know what you actually see, looking through a sextant. i know how they work now .. but i'm none the wiser. T_T but my one suggestion for your collection would be a "viking sun stone" (iceland spar). it's a natural crystal with very unique optical properties, and propably the earlist optical instrument ever used in naval navigation.

[@duanesmith5321]

Longitude easy at night For the Northern hemisphere, longitude for me is easy. If you can measure the Local Hour Angle of a star on the sky to an accuracy of 4 minutes or less you can get a longitude that is one degree or less from the observing position. A slight alteration of the calculation on page 279 of the Nautical Almanac and we have this. Longitude = GST - ( RA + LHA ) or LHA = GST - ( RA + Longitude) Where GST = Greenwich Siderial Time = GHA Aries. RA = the Right Ascension of the selected star. By using Polaris as the pivot point of a star clock, and a plumb bob string held against Polaris for a meridian reference, I've been getting longitudes to less than one degree easily from a list of 30 stars around Polaris. If measuring the LHA Westward, counter clockwise from the meridian upper branch, this is the angle to be added to the RA of the selected star. If measuring the LHA Eastward, clockwise, subtract LHA from RA. This gives Local Meridian Time, or Local Siderial Time. Then subtract LST from GST to get the GHA of the observing position. LHA can be measured in either time or degrees. This reduces to a minimum the time, math, materials, and paperwork needed to get something of a longitude, even if the observing position is not known. Just thought I would tell the world in case anyone is interested.

[@davidholder3207]

I recently finished reading the journal's of RN Captain Cook's first voyage round the world of 1678-1871. Quite often he refers to the ships position being derived from the average readings. I assume the average was derived from the several other ships officers who'd be taking sightings. Interestingly he would estimate the presence of a tidal current by the difference between where he thought the ship should be to where it really was. That shows the confidence he had with the calculating methods and instruments used at the time.

[@robertwilson8789]

Yes , using celestial in conjunction with DR (dead reckoning) that use tsd data (time , speed , distance) is generally considered an appropriate way to measure the effects of currents over slightly longer periods of time (6 hours or more) and in open seas .Not as effective in near coastal areas that can have wildly different currents .The preferred method for near coastal was the rdf (radio direction finder) triangulation method or triangulating navigation lights .

[@ibrahimkocaalioglu]

Nice information. Thank you very much.

[@feedme8991]

Great video! But dude, do something about the reflection in your glasses.

[@atomixfang]

great information, thanks!

[@karlschulte9231]

C'est tres bien mon ami! Very well done and intelligent blog. Karl WA2KBZ ham radio. Btw have been in your city and all but 3 prov. ( NWT Yukon and PEI. ). Up to Prince Albert. Mostly great. Exception was Calgary in Jan. Aux reviens!

[@ericsumma7654]

Just a couple of thoughts. 1: a normal sextant IS a bubble sextant. It uses the largest bubble in the world, the atmospheric bubble floating on the ocean. Measurements are taken within the bubble. 2: The rotation of the Earth is faster than 15°/sec. when referenced to stars and inertial references. It's only approximately 15°/sec when referenced to the sun resulting from the combined motions of the Earth's rotation and it's orbital motion around the sun. I'm not sure how this affects the use of a sextant. Thank you for the videos.

[@karhukivi]

None of it affects the use of a sextant because the measurements of angle and time are then used with astronomical tables (almanac) to get the position. The almanac has separate tables for the sun, moon, planets and stars. The sextant angle is referenced to the horizon, a tangent, so there is a correction for "dip". A bubble sextant for aircraft doesn't need that dip correction.

[@BipTunia_Microtonal_Cats]

14:45 Some stars must have been renamed. I've never heard of the star named Peacock. (near top left of the left page). I googled, it's now called Alpha Pavonis.

[@JPJPR]

Pavonis is the Latin genitive of Latin pavo = "peacock". It may refer to:- Pavonis Mons, a large mountain of Mars Pavo (constellation) Delta Pavonis, a nearby star (Source: Wiktionary)

[@gustavorad]

in the sextant the telescope inverts image because you have a convex lens in objetive and ocular. the other oculars have a concave lens, therefore they work as catalejo, and they give a little window not inverted.

[@brianhiles8164]

It is surmised by experts, that the popular image of the pirate with an eye patch, is supported by, or even because of, the matter of _solar foveal retinopathy_ -- partial blindness from looking at the Sun. The majority of speculation found online is the theory that navigators covered one eye to accrue dark adaptation -- night vision -- but I believe that a navigator of some experience would eventually develop partial blindness from looking directly into the Sun when taking a fix during the day with the Celestial Quadrant, so both factors were probably applicable.

[@CaptainGrief66]

That makes a lot of sense now, really neat fact

[@ddegn]

The Apollo astronaut assigned to take sextant readings would wear an eyepatch prior to using the sextant. The stars could be seen from the spacecraft but only when the eye is dark adapted.

[@steve1978ger]

I think the height of sextant development might be the one used in the Apollo program, which used rotary encoders directly linked into the Apollo Guidance Computer (AGC)

[@ddegn]

The Apollo sextant is why there are photos of astronauts wearing an eyepatch. The had to prepare an eye to see the stars when the spacecraft was in full daylight. Otherwise space looked intensely black when there eyes were adjusted to daylight.

[@rweems5796]

I hope you will read my comment- I’m curious where / what time period the trainer from which your celestial sphere was taken? My last name is Weems. PVH Weems was my great uncle. He, with Edwin Link, designed and built a nav trainer for WWII. My brother and I are on a hunt to find where these were built, more importantly, if any still exist. Saw your vid - thought I’d ask! I’ve no clue how you might contact me, hopefully there is a mechanism via You Tube. Thanks.

[@MatthewManire]

Have you created the video you mentioned, researching the WW1 engineer corps crew?

[@coptertim]

On 10 March 1947 a TWA Constellation NC90814, Star of Hollywood, left New York City at 1353 GMT en route to Geneva, Switzerland. At 0030 on the 11th, navigator George Hart stepped up into the plexiglass Astrodome to use his sextant when without warning, the dome failed and the explosive decompression pulled Hart out through the small window, killing him instantly. His body was never found. While explosive decompression can occure even today, this is the only example of an astrodome failure I can find.

[@yuglesstube]

Excellent! Thanks.

[@Allan_aka_RocKITEman]

@CanadianMacGyver >>> Great video...👍

[@bobcarry4820]

Thanks. I was always curious how a sextant works and you explained that and added that they do not work alone. They require extensive tables. Do they still print tables? GPS is very useful but may not always be available. I can use a compass though for my travels.

[@slome815]

Several nautical almanacs are still made every year. If you're just doing a basic noon reading that's not even necessary.

[@navret1707]

We used a periscope sextant in the P-3.

[@MrGus.1]

The US Navy ingeniously developed navigation forms which had many blanks which had to be filled in by the navigator using the nav pubs, basic nav data and the celestial measurements. The P-3 navigators were either nav trained pilots, or NFO's, but aboard ships, the navigation was very ably done by enlisted quartermasters. Take the sightings, fill in the blanks and establish your fix. All this theory is great, but all you need are the pubs, the sextant and basic nav data. The P-3A and P-3B could often detect subs, but often did not have an accurate nav solution making the position of the subject sub somewhat elusive.

[@WizzRacing]

One thing they also used was a watch, with second hand. As they needed to know their speed and distance travel.

[@ronaldjorgensen6839]

thank you

[@siggevibes]

Can't say I did understand that much, but this is fascinating nonetheless

[@IBITZEE]

did you realized when you were saying "...logitude lines (...) faster in Northern latitudes..." you were showing a map of the "South Pole"???!!! (yeah... I know... closing of longitude lines occur there too... but... ;-)) Nice video... keep it comming...

[@enginbaspinar4172]

Whats the music at the intro?

[@bozhijak]

If you have children who want to bust out of the boredom (necessary) have them see this. IMHO

[@morganahoff2242]

26:50 But...addition and division aren't commutative. Let's take a simple example, using two numbers that are easy to average in our heads: 6 and 12. 6 + 12 = 18, 18/2 = 9. Now divide first: 1/2 of 6 is 3, 1/2 of 12 is 6. 3 + 6 = 9. Okay, I see how they did that.

[@hugodesrosiers-plaisance3156]

I came for the sextant, je suis resté pour le noeud pap. Cheers.

[@bozhijak]

Point of order. Rotational presion ~ 24,000 yr?

[@SuV33358]

Thank golly for GPS

[@JSomerled]

I was trained in navigation by the USN.. before the days of gps

[@legendofnone3037]

I never considered how trenches were dug. I think that last sextant was cool, and the one taking 1/6 and adding them up together? It baffles me what humans are capable of. Necessity is the mother of invention, i guess

[@robrussell5329]

Kirk: "Mr. Spock, what would you say are our odds of getting out of this predicament?" Spock: " 7,439.6 to 1." Kirk: "7'439.6 to 1?" Spock: "I strive to be accurate."

[@CoolDudeClem]

I know it's immiture, but I can't hear the word "_sex_tant" without cracking a smirk. On the other hand though, I've allways been curious how these worked and how they were used.

[@hewhoadds]

20:30 wow. i want one

[@scaredofghosts6813]

The fact that sextants and octants isnt taught to highschoolers in algebra2/trig is mindboggling to me..blatant real life use of trig

[@feedme8991]

Interesting why the heck would you ever have the justified need to issue a (crude and imprecise) sextant to people operating on land in Western Europe of all places. I mean, WE had been properly mapped since forever and what's the realistic scenario where you: A. Had no idea where you are to the precision of like 100-500m off your military maps and B. and couldn't just do regular azimuth triangulation to obvious terrestrial reference points via a common engineering compass?

[@babaiishere]

I have original sextant I'm from India and I found lots of functions of this binocular and I have also found the see through the wall feature in the sextant and I want 100million go for this antique pls pin me I'll show you all I have

[@GenX-Grampa]

It’s all voodoo to me!😂 Thank God for GPS!

[@Justwantahover]

With hundreds of flat earth videos "explaining" how a sextant can only "work" on a flat earth, how come there are no flat earthers in the comment section, here??? 😂

[@sissyfus6181]

They're all over at MCTOON's $10,000 CN challenge crying.

[@danpfeiffer1649]

It's a shame the instrument you used as a demo is a $40 reproduction knockoff and not a real working sextant. I think you are getting to complex too fast in this explanation. To be fair, it takes more time than you have in a video like this. But the foundation concepts don't require an understanding of time, trig, or tables to explain. Start with that. Also at about 21 minutes you say lunar distance was all they had for 200 years and then in 1714 the longitude prize is created... Not quite. The data needed to make LD work (ephemeris) and the instruments needed to take the measurements (sextants) did NOT exist in 1514 or 1614 or really, even 1714. The development of LD was something that happened at the same time as chronometer development in the 1700's. It was something of a race actually. Read Dava Sobel's book "Longitude".

[@wiesejay]

One more and you’d have a sextet of sextants

[@474461]

So Earths navigated as Flat using Trig, so not a ball then, who knew.

[@sissyfus6181]

You did real well in school, didn't you?

[@iveneverseensuchbehaviorin5367]

not a single flerf understands what a sextant does

[@sissyfus6181]

@@iveneverseensuchbehaviorin5367 He's never navigated. They're like kids who can't do arithmetic trying to comment on calculus. A laughable bunch of yobs.

[@slome815]

Rofl, what? All celestial navigation tables require a round earth to work. There is no way to explain any kind of celectial navigation, especially when it comes to latitude, with a flat earth.

[@karhukivi]

Yes, but it is "spherical trig", not the stuff you should have done in school which is planar trig. In planar trig, the three angles of a triangle add up to 180 deg. In spherical trig, they can add up to 270 deg.

[@ГогоГого-э3ю]

Некоторая часть Аудитории ненавидят ваш наглосаксонский говор .... Ето как белка зудить, набравших орехами рот ... Их тошнить !!! ...

[@rautamiekka]

Way too hard for me.

[@iliyasmolla4362]

Hi

[@Louis-kk3to]

I would die at sea 🤢

[@sergeant5848]

I'll stick to the phone's GPS thanks.

[@skivvy3565]

Until find yourself without it for any reason, then what? 😉

[@sergeant5848]

@@skivvy3565 *Then* and only then, shall i grab the sextant out of the glove compartment of my vehicle, open and put on my reading spectacles, peruse the operations/owner's manual and learn how to use the darn thing properly at which point I shall be able to use my road maps book and orient it northerly in an attempt to reach my destination! 🤗

[@whatislifebro]

Earth is measured flat 📐💯

[@rescuedcarcollective4916]

Your video proves a flat earth!!😮

[@across_the_plane6800]

Hahaha this video and you are hilarious

[@dipokroy5427]

I have buyer ready Kolkata India