THE SOUTHWORTH PLANETARIUM 70 Falmouth Street Portland, Maine 04103 (207) 780-4249 usm.maine.edu/planet 43.6667° N 70.2667° W Founded January 1970 2022-2023: LXVII Sunrise: 6:08 a.m. Sunset: 5:37 p.m. Civil twilight begins: 5:39 a.m. Civil twilight ends: 6:06 p.m. Sun's host constellation: Aquarius Moon phase: *FULL MOON* (100% illuminated) Moonrise: 5:51 p.m. Moonset: 6:49 a.m. (03/08/23) Julian date: 2460011.29 "Common sense is the collection of prejudices collected by the age of eighteen." -Albert Einstein THE DAILY ASTRONOMER Tuesday, March 7, 2023 Inside Earth Consult any of the credible -and most of the illegitimate- dictionaries and you will find the following definition for astronomy: *astronomy (n): *the branch of science that deals with phenomena beyond Earth's atmosphere. [It is regrettable that 'astronomy' does not appear in Ambrose Bierce's 'Devil's Dictionary.] Despite this conventional definition, today's DA looks down and through the ground to answer an intriguing question related to our knowledge of Earth's complex interior. This question was prompted by recent news that Earth's inner core has either stopped rotating or has reversed its rotation direction altogether. *"I don't understand. We haven't even gotten to the bottom of the ocean, so how can we possibly know that Earth has a core or even what's inside our planet?" M.B.* Even though three people* have managed to travel to the Marianas Trench, the lowest point within the ocean, you ask a very good question. How can we possibly determine the sizes, locations and compositions of the zones within Earth’s interior despite not being able to deploy any probes through it? It is all done with seismic waves generated by Earthquakes. We can separate these waves into two types. - P waves, or primary/pressure waves - S waves, or secondary waves P waves are longitudinal compression waves, similar to the motion of a slinky. [Image credit: Narissa Spies] S waves exhibit a motion perpendicular to the direction of the motion, like that of a rope [Image credit: Wikimedia Commons] Whereas P waves travel along the direction of motion, S waves oscillate up and down along the motion path: [Image credit; Meghan Environmental Science] Here’s the rub: P waves can travel through solids, liquids and even gases, but S waves can only move through solids. Moreover, seismic waves travel at different velocities through different types of materials.** When an Earthquake -or another powerful wave-generating event such as a volcanic eruption- occurs, seismic waves propagate through Earth in all directions and can be detected by seismographs at other locations. By studying the seismographs throughout these locations, scientists can determine which waves are transmitted and the speed at which these waves travel. Since S waves cannot travel through liquid, they won’t be detected along any point within the ‘S-wave shadow zone, which extends along a 154-degree arc opposite the location of the seismic wave source (i.e. Earthquake) However, P waves are detected at all locations except for two more narrow “P shadow zone” regions. The refraction within the liquid outer core leaves gaps that produce these two shadow zones. [Image credits: US Geological Survey] Since Earthquakes are quite lamentably common, scientists have been able to quite accurately map the planet’s interior- the crust, mantle, liquid outer core and solid inner core- by analyzing both of these wave types generated by sources at multiple locations. I do hope that this explanation, though simplified, proves helpful. *Jacques Picard and Don Walsh (1960); James Cameron (yes, THAT James Cameron) (2012) *Sound waves also travel at different speeds through different media. For instance, sound travels at about 767 miles per hour through air at room temperature, but can travel at 10,200 miles per hour through copper. (Sound travels very quickly through metals. Consequently, one can hear a train approaching from quite a long distance merely by pressing an ear to a railroad track.) To subscribe or unsubscribe from the "Daily Astronomer" http://lists.maine.edu/cgi/wa?A0=DAILY-ASTRONOMER