<p>The book <em>The Planet Mars: A History of Observation and Discovery</em> by William Sheehan states that</p>
<blockquote>
<p>Herschel calculated the diameter of Mars to be 0.55 times that of the Earth, and he found its figure to be just as flattened as that of Jupiter---the ratio of the equatorial to the polar diameter he put at 16/15.</p>
</blockquote>
<p>How did he do these calculations? What experiments did he conduct to come to these conclusions?</p>

CC BY-SA 4.0
2024-04-19T10:43:34.713Z
No doubt he measured its angular diameter and then used known (computed from known orbit) distance to calculate diameter.
Leos Ondra
CC BY-SA 4.0
2024-04-20T01:59:02.313Z
@LeosOndra that's certainly likely, *but how?* Hopefully there's some written record where the methodology was explained. To estimate a diameter of a fuzzy, bobbing blob seen in an eyepiece, one needs some kind of calibration or reference. Did he use nearby stars in the same field with a known separation (were there any back then?), or hold his thumb out as a reference and open his other eye at the same time and view it by candlelight? For the orbit, did he use his own personal calculations, or was there an ephemeris available by that time? *Enquiring minds want to know!*
uhoh
CC BY-SA 4.0
2024-04-21T13:42:07.410Z
https://collection.sciencemuseumgroup.org.uk/objects/co56837/filar-micrometer-by-william-herschel-1780-1800-filar-micrometer
Leos Ondra
CC BY-SA 4.0
2024-04-21T14:57:52.010Z
@LeosOndra *very nice!*
uhoh
<observational-astronomy><history><mars>
How did Herschel determine the diameter of Mars?
usernumber
# How did Herschel determine the diameter of Mars?

(Asked by: username_0 on 2024-04-19)

The book *The Planet Mars: A History of Observation and Discovery* by William Sheehan states that

> Herschel calculated the diameter of Mars to be 0.55 times that of the Earth, and he found its figure to be just as flattened as that of Jupiter---the ratio of the equatorial to the polar diameter he put at 16/15.

How did he do these calculations? What experiments did he conduct to come to these conclusions?

**Tags:** observational-astronomy, history, mars

> **Comments:**
> **username_1**: No doubt he measured its angular diameter and then used known (computed from known orbit) distance to calculate diameter.  
> **username_2**: @username_1 that's certainly likely, *but how?* Hopefully there's some written record where the methodology was explained. To estimate a diameter of a fuzzy, bobbing blob seen in an eyepiece, one needs some kind of calibration or reference. Did he use nearby stars in the same field with a known separation (were there any back then?), or hold his thumb out as a reference and open his other eye at the same time and view it by candlelight? For the orbit, did he use his own personal calculations, or was there an ephemeris available by that time? *Enquiring minds want to know!*  
> **username_1**: https://collection.sciencemuseumgroup.org.uk/objects/co56837/filar-micrometer-by-william-herschel-1780-1800-filar-micrometer  
> **username_2**: @username_1 *very nice!*  

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## 0 answers

