Home One is the cruiser of Admiral Ackbar in Return of the Jedi. Its official length is stated at about 1300 m in nearly all official sources, although it's sometimes 1200 m or even 1400 m.

These numbers are wrong.

We know because the size is larger in the movie itself.
Several measurements have been done over the years, most notably those of astrophysicist Curtis Saxton, published in the Star Wars Technical Commentaries website: http://www.theforce.net/SWTC/mcc.html#dimensions
Comparisons of the Home One with other ships surrounding the cruiser or inside its hangar bay gives a length of more than 3 km. Here is a summary:
However, these were made a long time ago, and the reference images had poor resolution. A key resource was this photomosaic constructed by Frank Gerratana:
The frames that went into that mosaic come from an impressive shot from the sequence where the Rebel fleet jumps into hyperspace. For a few seconds, the camera pans over the entire length of the cruiser, which acts as a magnificent backdrop as ships go by:
Most of Saxton's measurements are focused on the hangar size, especially the height necessary for the Tydirium shuttle to be able to fly through the entrance
Given the height of the shuttle, it's easy to calculate the necessary size of the hangar, and from there the minimum size of the cruiser to keep the proportions intact. Wait, I said easy? No, there are several caveats.
The shuttle was not a single object in the filming: there was the miniature model to show the ship in motion in space, there were detailed matte paintings when landed (pictured), and also the full-size ramp and entrance for actors to interact with. And there are inconsistencies.
Specifically, the size and shape of the upper wing doesn't exactly match among different versions. And this is the key in all comparisons involving the shuttle.
Furthermore, the width of the hangar platform and interior is not straightforward, so complicated measurements have to be done on a matte painting. And the best orthogonal view of the cruiser with the outside hangar opening has low resolution:
I do trust these measurements, but the compounded uncertainties ask for an independent confirmation. And there is! In the summary, one result says "corvette comparison", providing a minimum Home One length of 3.15 km. How was this done?
Well, in the sequence showing the jump to hyperspace, one ship seems to be moving slowly in front of the cruiser, perfectly positioned for a comparison. It appears in the photomosaic above, and here:
When Saxton made his measurements, he interpreted it as a 150 meters long Corellian corvette, or Rebel blockade runner, and the proportion with the photomosaic scaled the cruiser to a minimum of 3.15 km as explained before.

HOWEVER, this is *not* a Corellian corvette!
This ship was a different model, and has since been dug out of obscurity and given the names Braha'tok-class or Dornean gunship. It was later featured in Rebels and Rogue One.
And has official length of 90 meters, which throws the scale off.
https://starwars.fandom.com/wiki/Braha%27tok-class_gunship
It's easy to just downscale the minimum size of the Home One in the same proportion to match the new size, but… The calculated size was based on an old, low-resolution photomosaic.
And now we have digital, HD versions of the film… https://twitter.com/DarkSapiens/status/1300200088405446663
So.

I made a new photomosaic.

Before Twitter compression, the Home One in this image is more than 12000 pixels long.
In each frame used, I carefully removed all other ships to avoid confusing the photomerge automated procedure, and painted the stars black so they wouldn't be used in the tracking. The gaps are where I couldn't find frames that showed that section unobstructed
There are two moments where most of the view is hidden by a foreground ship. First a Rebel transport, then the Millennium Falcon. Can be seen here:
This is important because by the time the obstruction is out of the way, the camera has moved to the right and our perspective of that section of Home One has changed. Bits of the cruiser closer to the camera move faster to the left than further ones.
Which means frames of the same section of the cruiser from different moments of the video no longer match, and Photoshop produced three sections that I had to join by hand. There's overlap, but these perspective issues make it difficult.
In my final result, Home One is 12070 pixels long. But a previous attempt gave 12216 pix, for example. 100-200 pixels won't matter much for my argument, but since my claim is that the cruiser is larger, I'll be as conservative as possible and use only 12000 when comparing.
So. The Dornean gunship is 570 pixels long. If 570 pix are 90 m, then 12000 pix are 1895 meters.

If the gunship was at the same distance from the camera as the cruiser silhouette, which means it'd have to be *inside* the cruiser, then Home One would be 1,9 km.
The Dornean gunship is closer to the camera than Home One, and we've seen there is perspective in the shot so closer objects appear bigger, so we have our minimum size: the Home One Mon Calamari cruiser is larger than 1.9 km if the Dornan gunship is 90 m long.
However, is the Dornean gunship 90 m long? If my claim is that the official sources for the length of Home One are wrong, why am I trusting them for the gunship?

Well, one could say that *if* a source lists its length as 90 m, then can't *also* say Home One is less than 1.9 km
But we can measure other things! For example, let's find one of the frames where the Millennium Falcon is as far from the camera as possible. Here the Falcon is just 233 pixels long. And its official length is about 34.5 meters.

This gives a Home One size of more than 1777 m.
There are also X-wings, flying in a somewhat straighter path while Lando's piloting his old ship from side to side and being flashy.
This is the further one I could find, just 120 pixels long. With an official length of 13.4 m, the minimum size for Home One is… 1340 meters!
This one's in close to the official size! However, as stated before, the X-wings would need to be flying *inside* the cruiser for the distance to the camera to be the same. This particular fighter is flying in front of the gunship, which means it's even closer, as expected.
We can also do some consistency checks. If that X-wing is at the exact same distance as the gunship (which can't be because it's flying in front of it), then the gunship would be 64 meters long.
The Millennium falcon seems to fly directly below the gunship, because if in the frame I used it was at the exact same distance, then the gunship would be 84 meters long. Which is very close to the official size of 90 meters.
But why has the 1200-1400 m size stuck for so long? For this part, we might have to look at the other model constructed to film Mon Calamari cruisers in the film. The so-called Liberty-type, from the most famous representative:
This miniature model actually played *two* different cruisers in the movie. First it was similar to Home One, grey and wingless
Then, to add variety, they attached wings to it and painted it in a more organic pattern.
Both versions of this model can be seen exchanging turbolasers with Star Destroyers and are slightly smaller, with a Star Destroyer supposed to be 1 mile (1.6 km) long:
So perhaps the size for the Liberty-types stuck for all Mon Cal cruisers, and the larger size of Home One in the movie was forgotten or went unnoticed.
However, there's one last thing: was it supposed to be larger? Or are we making too much of a fuss out of a shot that might have been composed like that to make it visually impressive, rather than physically accurate? What if it was just a mistake in the filming?
After all, there's this other shot in Return of the Jedi that either implies there's a gigantic Nebulon-B frigate, or a tiny Corellian corvette… or a mistake in the order of the layers. The relative motion suggests the corvette here should've been *behind* the frigate (left).
A clue to the different sizes of the cruisers can be found in the hangars. Home One has that small opening on starboard, while Liberty has a sizable ventral hangar, proportionally larger.
However in this shot the Liberty-type is either *very* close to that Star Destroyer, or it's meant to represent a larger size.

Here I believe that the variety of shapes in the Mon Cal model allowed them to shoot a lot of close-ups representing more than 2 different cruisers
But the last point I'll make is this: take a look at the start of the sequence with the Rebel fleet about to jump. There's the Home One on the left, and a wingless Liberty-type on the right. Notice their motion.
They both start at a similar apparent size, but as the camera moves forward it overtakes the Liberty-type, but the Home One barely moves
Smaller ships like frigates, corvettes and transports also seem to be moving forward, slightly slower than the camera. So either the Home One is moving forward with the camera, much faster than the Liberty-type… or it's moving at a similar speed and is much larger.
In the panning shot along the Home One, we see the gunship and transports also moving faster than the cruiser. Since they kept this consistent between shots, I believe the sequence implies that the *intent* of the filmmakers was to make the Home One a larger cruiser.

Fin :)
I must say I'm always incredibly amused when the actual size of Home One appears in fan-created mods of videogames. They seem to just ignore the official length in favor of the movie-accurate one 😄
It needs to be noted that the size of Home One in the original Empire at War game was about 3.2 km rather than 1.3 km, however! I always laugh with this, haha
So anyway, here's the promised thread. I know @AdmiralNick22 has already seen it, and perhaps it would be of interest to @TheSpaceshipper, @visor_t, @EckhartsLadder, @LukeTogniMU, @mel_miniatures and others…
You can follow @DarkSapiens.
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