Editing 2971: Celestial Event

{{comic
| number    = 2971
| date      = August 12, 2024
| title     = Celestial Event
| image     = celestial_event_2x.png
| imagesize = 471x300px
| noexpand  = true
| titletext = If we can get a brood of 13-year cicadas going, we might have a chance at making this happen before the oceans evaporate under the expanding sun.
}}

==Explanation==
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[[Randall]] has often obsessed over celestial events he wishes to see, and here lists three of these in this comic, hence the title. Recently there have been two total {{w|solar eclipse}}s in the US (2017 and 2024), so that has been ticked off his bucket list. And there have been {{w|aurora|northern lights}} this year far down into the mainland US. So it is likely he has also seen those. Whether or not he has seen a {{w|comet}} visible to the naked eye is not known, but there have been some candidates in his lifetime.

The next two on the list are not celestial events, but the lack of cloud cover is relevant in order to see the first three. Finally, he mentions the emergence of 17-year {{w|Periodical cicadas|cicadas}} which occurred most recently in the midwestern US in 2024 and in the eastern US in 2021. 

An extra special celestial event would be to see all the three first things at the same time during a 17-year cicada event. Northern lights and a comet can only be seen during the night, except during a total solar eclipse where they could be visible for the few minutes of totality. (However, it rarely gets really dark during totality, and it is so short that night vision cannot be achieved before the totality is over.)

This joke of the comic comes from Randall multiplying the fraction of time that these selected classes of events occur over a particular location on the Earth's surface, in this case, {{w|Cambridge, Massachusetts}}, where the cartoonist was living when this comic was published. The resulting product is the expected frequency that all of them would occur at the same time at that location. The value he calculates is once every 4.3 billion years. This is in the same ballpark as the current age of the Earth, about 4.5 billion years.

The calculation itself is not going to be accurate, which is likely part of the joke. Multiplying probabilities only works for random variables that are entirely independent. If nothing else, orbits are (luckily) not random{{cn}}. It also requires that all of the probabilities remain constant over time. In reality, cicadas will not exist for very long compared to the time scale, since Earth will become uninhabitable to complex life within a billion years' time and all life will be extinct within [https://en.wikipedia.org/wiki/Future_of_Earth 4 billion years]. Also, the moon is moving away from Earth, and total solar eclipses will cease to occur in about [https://www.space.com/37627-total-solar-eclipse-earth-moon-alignment-future.html 600 million years].

Conversion of "days" and "months" to fractional years, required for conservation of units in the equation, is ambiguous as presented due to the leap year phenomenon and the inconsistent number of days in a month. Differing values for these fractional years yield a range of frequency solutions between 4.2 and 4.4 billion years. If the value for days in a year is given as 365.25 (the mean value for all years, ignoring infrequent additional 'leap year' corrections), as in the first term of the equation ((20/365.25)/11), and the mean value for days in two months is given as 60.9 as in the second term ((60.9/365.25)/50), the result is 4.2995 *10^9 years.

In the title text, Randall mentioned 13-year cicadas. Massachusetts is near the northern limit of {{w|Periodical_cicadas|<em>Magicicada</em>}} distribution, and only one 17-year brood is established there (and not in Cambridge, MA). In other parts of the US, a 13-year brood and a 17-year brood emerged during 2024, an event that only happens once every 13x17 = 221 years. This caused a lot of noise and double the amount of dead cicadas after they had mated. Randall wondered whether it would be possible to get a brood of 13 year cicadas started near his home. In that case he could replace 13 with 17 in the calculation. Then, he hopes to achieve his "really spectacular show" before Earth's oceans evaporate. However, 4.3 billion years x13/17 is still more than 3 billion years. The oceans will not last that long, and given the merely 40 million year history of the genus <em>Magicicada</em> and its relatives, neither will the periodical cicadas.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:<u>Approximate frequency in my area</u>

:Active northern lights: 20 days per solar cycle

:A naked-eye "Great Comet": 2 months every 50 years

:Total eclipse: once every 350 years

:Clear skies: 50% of the time

:17-year cicada emergence: 2 months every 17 years

opening bracket

20 days over 11 years multiplied by 

2 months over 50 years multiplied by

1 over 350 years multiplied by

one half multiplied by

2 months over 17 years 

closing bracket to the power of -1

equals 4.3 billion years

:[Caption below the panel:]
:Every 4 billion years or so, my neighborhood gets to see a ''really'' spectacular show.

{{comic discussion}}

[[Category:Astronomy]]
[[Category:Solar eclipses]]
[[Category:Time]]
[[Category:Animals]]