<p>Such transit events occur when Earth and Mars are simultaneously at a node where their orbital planes intersect.</p>

<p>Between transits at the <em>same</em> node, both Earth and Mars must complete whole numbers of orbits, where 1 Mars year = 1.8808 Earth years.
<a href="https://ui.adsabs.harvard.edu/abs/1983JBAA...93..120M/abstract" rel="noreferrer">Meeus and Goffin 1983</a> identify patterns of 79-year and 284-year intervals:</p>

<p>79.0 Earth years &approx; 42.0 Mars years
<br/>284.0 Earth years &approx; 151.0 Mars years</p>

<p>Their Table II arranges the May and November transit dates in separate "panoramas" with 284 years between rows and 79 years between columns.
These are somewhat analogous to a <a href="https://eclipse.gsfc.nasa.gov/SEsaros/SEperiodicity.html#section109" rel="noreferrer">saros-inex panorama</a> of solar eclipses.</p>

<p>Between transits at <em>opposite</em> nodes, both Earth and Mars must complete odd numbers of half-orbits:</p>

<p>Nov&rarr;May: 25.5 Earth years &approx; 13.56 Mars years
<br/>May&rarr;Nov: 100.5 Earth years &approx; 53.43 Mars years</p>

<p>These Mars year fractions differ from 0.5 because the half-orbit near perihelion takes significantly less time than the half near aphelion.
Meeus and Goffin Table&nbsp;I also shows some intervals of 21.5 and 53.5 years outside the present few centuries.</p>

CC BY-SA 4.0
2019-08-11T22:54:14.620Z
Very nice answer, thanks!
uhoh
CC BY-SA 4.0
2019-08-11T19:32:05.497Z
2019-08-11T19:32:05.497Z
Mike G
<p>According to <a href="https://astronomy.stackexchange.com/a/26178/7982">this answer</a> transits of Earth across the disk of the Sun as seen from Mars:</p>

<blockquote>
  <p>The (Arthur C.) Clarke story is quite correct. Earth (and Moon) would transit the sun from the viewpoint of Mars on 8 May 1905, 11 May 1984 and again on 10 Nov 2084.</p>
</blockquote>

<p>no surprises there, but they</p>

<blockquote>
  <p>...follow a pattern of occurring after 26, 79 then 100 years.</p>
</blockquote>

<p>Why is that exactly? Why this pattern?</p>

CC BY-SA 4.0
2019-08-11T03:09:05.777Z
The Wikipedia addresses this, but not in much detail.   https://en.wikipedia.org/wiki/Transit_of_Earth_from_Mars#Conjunctions
userLTK
<orbit><earth><mars><planetary-transits>
Why do transits of Earth across the Sun seen from Mars follow a pattern of occurring after 26, 79 then 100 years?
uhoh
# Why do transits of Earth across the Sun seen from Mars follow a pattern of occurring after 26, 79 then 100 years?

(Asked by: username_0 on 2019-08-11)

According to [this answer](https://astronomy.stackexchange.com/a/26178/7982) transits of Earth across the disk of the Sun as seen from Mars:

> The (Arthur C.) Clarke story is quite correct. Earth (and Moon) would transit the sun from the viewpoint of Mars on 8 May 1905, 11 May 1984 and again on 10 Nov 2084.

no surprises there, but they

> ...follow a pattern of occurring after 26, 79 then 100 years.

Why is that exactly? Why this pattern?

**Tags:** orbit, earth, mars, planetary-transits
> **Comments:**
> **username_1**: The Wikipedia addresses this, but not in much detail.   https://en.wikipedia.org/wiki/Transit_of_Earth_from_Mars#Conjunctions  

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## 1 answers

### Answer by username_2 on 2019-08-11. Score: 10 | Accepted answer

Such transit events occur when Earth and Mars are simultaneously at a node where their orbital planes intersect.

Between transits at the *same* node, both Earth and Mars must complete whole numbers of orbits, where 1 Mars year = 1.8808 Earth years.
[Meeus and Goffin 1983](https://ui.adsabs.harvard.edu/abs/1983JBAA...93..120M/abstract) identify patterns of 79-year and 284-year intervals:

79.0 Earth years ≈ 42.0 Mars years
  
284.0 Earth years ≈ 151.0 Mars years

Their Table II arranges the May and November transit dates in separate "panoramas" with 284 years between rows and 79 years between columns.
These are somewhat analogous to a [saros-inex panorama](https://eclipse.gsfc.nasa.gov/SEsaros/SEperiodicity.html#section109) of solar eclipses.

Between transits at *opposite* nodes, both Earth and Mars must complete odd numbers of half-orbits:

Nov→May: 25.5 Earth years ≈ 13.56 Mars years
  
May→Nov: 100.5 Earth years ≈ 53.43 Mars years

These Mars year fractions differ from 0.5 because the half-orbit near perihelion takes significantly less time than the half near aphelion.
Meeus and Goffin Table I also shows some intervals of 21.5 and 53.5 years outside the present few centuries.

> **Comments:**
> **username_0**: Very nice answer, thanks!  

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