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tl;dr:

Both

quote Wikipedia to state that the Apollo 13 astronauts reached 400,171 km from Earth.

I'm not able to reproduce that value.


According to JPL's Horizons the distance between the center of the Earth and the center of the Moon at the beginning and end of occultation times from here are:

          Event                        UTC        Earth-Moon R (km)
-------------------------  --------------------   -----------------
Lunar occultation entered  1970-Apr-15 00:21:00       404418.652
Lunar occultation exited   1970-Apr-15 00:46:00       404423.901

JPL Horizons screenshot

Just in case I entered something wrong using the Horizons interface, I've double checked the numbers with Skyfield on DE421 and go the same thing except for about 100 meters because the times scales are slightly different due to accumulated leap seconds. Note these distances are not light time or otherwise corrected, they are just the differences in positions.

Let's call the Earth-Moon distance at the midpoint time to be 404 421.3 km, and the equatorial radii of the Earth and The Moon 6378.1 km and 1738.1 km.

Wikipedia's Apollo 13 (quoted in this answer) says:

The flight passed the far side of the Moon at an altitude of 254 kilometers (137 nautical miles) above the lunar surface, and 400,171 km (248,655 mi) from Earth, a spaceflight record marking the farthest humans have ever traveled from Earth.

Wikipedia's List of spaceflight records: Speed and altitude records (quoted in this answer) says:

Farthest humans from Earth

The Apollo 13 crew (Jim Lovell, Fred Haise, and Jack Swigert), while passing over the far side of the moon at an altitude of 254 km (158 mi) from the lunar surface, were 400,171 km (248,655 mi) from Earth. This record-breaking distance was reached at 0:21 UTC on 15 April 1970.

Question: I don't know how to combine the Horizons distances, the radii of the Earth and Moon, and the reported altitude in order to get to 400,171 km. Can someone help?

Secondary item is that the 2nd Wikipedia quote gives the maximum at 00:21 UTC but that seems to be the time of entry into occultation, not the midpoint 12.5 minutes later, that could be a clue, or just an oversight.

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    $\begingroup$ @OrganicMarble max Earth altitude: 404421 - 6378 + 1738 + 254 = 400035, max distance from Earth center: 404421 + 1738 + 254 = 406413 So there's perhaps a 136 kilometer disparity with Wikipedia $\endgroup$
    – uhoh
    Dec 31, 2018 at 1:37
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    $\begingroup$ Wow, that 1970 datestamp is annoying - my programmer brain has been trained to immediately suspect it as wrong/a bug 😂 $\endgroup$ Dec 31, 2018 at 17:06
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    $\begingroup$ @LightnessRacesinOrbit it's less problematic for people who started to program "way back" in the 1900's. $\endgroup$
    – uhoh
    Dec 31, 2018 at 17:18
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    $\begingroup$ @uhoh Alan Shepard on his first mission probably won the US award for spaceflight travel the farthest from the moon.... $\endgroup$ Jan 1, 2019 at 2:26
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    $\begingroup$ Yeah, I meant the Mercury one. $\endgroup$ Jan 1, 2019 at 2:36

3 Answers 3

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The Wikipedia article on Apollo 13 disagrees with itself. In the main text of the article it says:

At pericynthion, Apollo 13 set the record (per the Guinness Book of World Records), which still stands, for the highest absolute altitude attained by a crewed spacecraft: 400,171 kilometers (248,655 mi) from Earth at 7:21 pm EST, April 14 (00:21:00 UTC April 15).[115][note 4]

However, note 4 says:

A reconstruction of the trajectory by astrodynamicist Daniel Adamo in 2009 records the furthest distance as 400,046 kilometers (248,577 mi) at 7:34 pm EST (00:34:13 UTC).

I hacked my plotting script in this answer to plot the altitude of a satellite. Here's a plot of the Moon over the time that Apollo 13 was occulted by it (0:21:35 to 0:46:19 GMT). This plot uses TDB, which was 40.46 seconds ahead of GMT / UTC.

Moon altitude plot

So the altitude of the centre of the Moon above the surface of the Earth at 1970-Apr-15 0:35 TDB was ~398046.1 km. If we add that to the lunar equatorial radius of 1738.1 km and the Apollo 13 lunar altitude of 254 km we get a total of 400038.2 km, which is within 8 km of Daniel Adamo's value.


Here's a table of all the quantities of interest calculated by the script at 0:35 April 15 TDB.

Quantity Value
Distance 404421.52 km
Moon altitude 398045.98 km
Earth radius 6375.53 km
Declination 20.37°
Orbit speed 966.6 m/s
Earth speed 435.8 m/s
Ground speed 577.7 m/s
  • Distance is the centre to centre distance between the Earth and the Moon.
  • Moon altitude is the distance from the centre of the Moon to the sublunar point (on the surface of the Earth).
  • Earth radius is the distance from the sublunar point to the centre of the Earth ellipsoid.
  • Declination is the declination of the Moon, which is equal to the geocentric latitude of the sublunar point (not the usual geodetic latitude).
  • Orbit speed is the Moon's orbit speed relative to the centre of the Earth.
  • Earth speed is the Earth's rotation speed at the sublunar point.
  • Ground speed is the speed of the sublunar point relative to the ground.
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    $\begingroup$ Thanks for the link to my former co-worker's page! Dan was briefly my boss after one of the many re-orgs in the SMS. $\endgroup$ Nov 28, 2022 at 12:47
  • $\begingroup$ The present answer doesn't repeat it explicitly, but the linked answer implies that the plotting script takes into account that the Moon does not orbit in the Earth's equatorial plane. If I'm reading this right, it determines the sublunar point on the Earth ellipsoid (which is the right thing to do, rather than simply using the equatorial radius of the Earth). $\endgroup$
    – GNiklasch
    Nov 29, 2022 at 15:44
  • $\begingroup$ @GNiklasch Yes, that's right, my calculation computes the altitude of the centre of the Moon relative to the sublunar point on the WGS84 ellipsoid (at nominal sea level). $\endgroup$
    – PM 2Ring
    Nov 29, 2022 at 16:27
  • $\begingroup$ It's obvious to me that I don't have the orbital elements knowledge that you have. How can the trajectory according to the NASA article be hyperbolic ? (see my answer). And could you say from the shown values in the article what the used plane of reference is ? $\endgroup$
    – Cornelis
    Dec 3, 2022 at 14:06
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    $\begingroup$ @Cornelis I get the position vector of the centre of the Moon relative to the centre of the Earth from Horizons. And then I calculate the radius (6375.5 km) at the point where that Moon-Earth line intersects the surface of the Earth (actually the WGS84 ellipsoid). $\endgroup$
    – PM 2Ring
    Dec 4, 2022 at 13:13
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Let's start with the easy bits. The distance from the center of the Moon at the far point. The max distance that the Moon was from Earth was actually at the exit point. The exact distance involves some complex geometry. Another thing to keep in mind is the Earth and Moon are not exact spheres, the radius can vary quite a bit. The Earth's actual radius can vary between 6353-6384 km depending on exactly where you are, it is less towards the poles. The Moon's similarly can vary between 1736-1738 km. Thus whatever value we give could be off as much as 33 km, depending on which point of Earth/ the Moon we are talking about.

The distance should be (dMoon_Earth)-rE+rM+dA13. Let's use the values that will give the largest number for all of these, and we get 400,061 km, which is still less then the reported value. This would assume the min distance from the Equator of Earth (Unlikely) and the equator of the Moon (Likely).

Interestingly enough, the NASA article cites the Guinness Book of World Records as the source it uses. Other sources give even higher numbers, such as 401,056 km.

My guess is that they picked the distance to a point on the Earth, but not necessarily the closest point to Earth. A point on Earth that wasn't directly in the line of closest approach would give a slightly larger value, as is seen.

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  • $\begingroup$ As this is a puzzler and has survived for two rotations of the Earth without explanation, educated guessing is welcome. Britannica's num is very close to what I got (404421 - 6378 + 1738 + 254 = 400035) and that's encouraging, and if you use Earth's polar radius you get exactly what they get! I think you may be very close to a solution; Wikipedia is wrong. $\endgroup$
    – uhoh
    Jan 2, 2019 at 2:39
  • $\begingroup$ They cite NASA, who comes the Guinness book if world record. Note that the Britanica value is off by 1000 km. $\endgroup$
    – PearsonArtPhoto
    Jan 2, 2019 at 2:49
  • $\begingroup$ Oh! I was so focused on the last few digits I missed the thousands. Okay, mystery not solved... yet ;-) $\endgroup$
    – uhoh
    Jan 2, 2019 at 2:50
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    $\begingroup$ I actually did the first time I saw it too, then looked a bit more careful... $\endgroup$
    – PearsonArtPhoto
    Jan 2, 2019 at 3:11
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Flight data
Screenshot of the Apollo 13 trajectory's orbital elements around the Moon, published in NASA's article Apollo 13 Moon View Using LRO Data.
Here we see that the shortest distance of the Apollo crew to the Moon's centre (pericynthion) was 1988.8 km on April 15, 1970, 00:33:57 UT.
(According to Wikipedia a reconstruction of the trajectory by Daniel Adamo records the furthest distance at 00:34:13 UTC)

Horizon data

Above is a screenshot of the ephemeris (R.A. and DEC) of the Moon produced by Horizons Web Application also showing the distance from Earth to the Moon (delta).
It shows that on April 15,1970 00:34:00 UT that distance was 404460 km.

The screenshot also shows that at that time the declination of the Moon was about 20.3⁰ so the shortest distance to the Moon on Earth's surface happened at 20.3⁰ latitude.
From a diagram in Wikipedia's article about Earth's radius we can deduce that at 20.3⁰ latitude the geocentric radius is about 6376 km.

Conclusion: the distance from Earth to Apollo 13 was 1989 + 404460 - 6376 = 400073 km.

On April 15,1970, the right ascension (RA) of the Sun and the Moon were 1 h, 33 min. and 8 h, 53 min. respectively.
So for the prime meridian of Greenwich on April 15, 00:00:00 UT the RA would have been 13 h, 33 min. and thus at 00:34 UT, with the Apollo 13 right behind the Moon, the Greenwich meridian RA was 14 h, 07 min.
The difference between this RA and that of the Moon is 5 h, 14 min. and this means that the sublunar point at April 15, 00:34 UT was at 20.3⁰ N, 78.5⁰ W., near the southwest coast of Cuba.

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    $\begingroup$ This is great, thank you! Currently we can't split bounties so I'll add a 2nd one for PM2Ring's answer. SE requires the next one to be at least double this one; when I bounty two helpful answers I give the higher one to the use with the lower rep. $\endgroup$
    – uhoh
    Dec 4, 2022 at 23:12
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    $\begingroup$ @uhoh That's very generous, thank you too, I will spend this bounty well ! A fine puzzler that gave me the urge to search for the truth ! $\endgroup$
    – Cornelis
    Dec 5, 2022 at 9:39

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