Editing 2925: Earth Formation Site

{{comic
| number    = 2925
| date      = April 26, 2024
| title     = Earth Formation Site
| image     = earth_formation_site_2x.png
| imagesize = 478x521px
| noexpand  = true
| titletext = It's not far from the sign marking the exact latitude and longitude of the Earth's core.
}}

==Explanation==
[[Cueball]] reads an historical marker celebrating the formation of the {{w|Earth}}. As with most historical markers, it claims to rest on the exact spot of the event, that the Earth formed in this specific location. It also specifies the Earth's formation to the precise year 4.45 billion ({{w|Long and short scales|4,450 million}}) BCE.

The absurdity of the sign is threefold:
# The Earth did not form on its surface
# The precise year of the Earth's formation is not knowable
# Historical markers typically refer to events within the past several centuries

'''#1: The Earth did not form on its surface'''

First, the Earth formed at its center, not anywhere on its surface, so an “Earth formed here” sign on the surface is amusingly incorrect.

One may argue that technically the sign is ''above'' the right spot, just as every location on Earth is above the right spot. However, the sign refers to “this location,” not to a spot underground.

Alternately, perhaps it describes the point at which {{w|Theia (planet)|Theia}} made its first impact point (to form, from the pre-impact Earth, the composite body that we more recognise as the body that was the post-impact one), although this spot would have been ''vastly'' disturbed by the collision, and even somehow keeping tabs on its theoretical position by reference to its (relatively unaffected) antipode would be confounded by all the subsequent tectonic movement and volcanic refreshing/rearranging of almost the entire planetary crust over the next four thousand million years or so.

If an omniscient observer wanted to mark the spot in space where the Earth started forming, they would need an historical marker floating in space, not on the surface of the (moving) Earth. That’s due to the {{w|Sun#Motion|Sun's 225-million year long orbit around the center of the}} {{w|Milky Way galaxy}} and the movement of the galaxy itself through space relative to other objects.

'''#2: The precise year of the Earth's formation is not knowable'''

Secondly, specifying a single year of formation BCE (Before the Common Era) is an amusingly precise choice. It takes [https://science.nasa.gov/exoplanets/how-do-planets-form/ tens or hundreds of millions of years] for a planet to form. Picking a year would require some specific definition of when a gradually-coalescing mass of proto-planet dust and gas could be considered a planet, as well as the impossible ability to determine when that mass met the definition.

If the precise year was knowable, the probability of the number actually ending in seven consecutive zeros would be on the order of one-in-ten-million.

The topic of what precisely makes a planet — related to the 2006 redefinition of a planet and the subsequent reclassification of Pluto from the solar system's ninth planet to a dwarf planet — has been covered before in [[473: Still Raw]] and referenced in other XKCD comics.

There is some poetry in the idea that there was a precise year, some 4.45 billion years ago, that was the first true year, the first Earth orbit around the sun. By definition, the Earth is the same age as the number of Earth orbits that have ever taken place.

Note: The date shown for the formation of the Earth, 4.45 billion years ago, also differs from the commonly accepted date, [https://pubs.usgs.gov/gip/geotime/age.html 4.54 (±0.05) billion years]. The difference lies in the transposition of two digits, 5 and 4, potentially a mistake, as is common in historical markers.

'''#3: Historical markers typically refer to events within the past several centuries'''

Historical markers are placed at precise locations where historical, religious, and even mythological events are believed to have happened — such as where battles have been fought, or where famous people resided or accomplished something. Typically these signs refer to more recent events.

'''The title text'''

The title text refers to the 'exact latitude and longitude of the Earth's core,’ Of course, since the lat-long geographic coordinate system is used for locating places on the ''surface'' on the Earth, the center of the Earth does not have latitude and longitude.

Also, a historical marker referring to the Earth’s core could be placed anywhere on the surface, and its specific location in the real world wouldn’t mean anything; just as before, every spot on Earth is above the right spot.

Examples of similar historical marker signs:
* {{w|File:Equator sign kenya.jpg|Kenya Equator latitude}}
* {{w|File:Arctic Circle sign.jpg|Arctic Circle latitude}}
* {{w|File:Prime meridian.jpg|Prime Meridian longitude}}
* {{w|File:2022-06-06 18 39 21 Sign reading "Highest Point on Interstate 80 East of the Mississippi River" along eastbound Interstate 80 (Keystone Shortway) just east of Exit 111 in Pine Township, Clearfield County, Pennsylvania.jpg|Highest Point}}
* {{w|File:World's lowest point (1971).jpg|Lowest Point}}

'''Analysis'''

The comic and its title text are actually inverse jokes of each other: The historical marker in the comic assigns an attribute of the Earth’s '''center''' (the site of formation) to a spot on its '''surface''', while the historical marker mentioned in the title text assigns an attribute of '''surface''' locations (latitude and longitude) to the Earth’s '''center'''.

==Transcript==
{{incomplete transcript|Generated by EARTH — Do NOT delete this tag too soon.}}

:[Cueball is standing in front of a sign in a field of grass. Rocks and plants are scattered across the ground. The sign reads:] 
:HISTORICAL MARKER
:<big>EARTH</big>
:FORMATION SITE
:--- 4,450,000,000 BCE ---
:At this location in the year 4,450,000,000 BCE, a cloud of dust and gas gravitationally collapsed to form the Earth.

{{comic discussion}}

[[Category:Comics featuring Cueball]]
[[Category:Geology]]