The other kind of tides
Earth tides flex the planets upper crust daily
Living on Canada's East Coast, we are all familiar with the daily rise and fall of the ocean tides along our shores; it's part of the charm of living along the ocean's edge.
But, did you know that there is also such a thing as an Earth tide?
This natural phenomenon is the actual flexing of the Earth's thin, upper crust due to the gravitational pull of the sun and moon.
Much as the ocean's waters respond to the gravitational pull of the sun and moon, rising and falling twice daily, so, too, does the thin, outer layer (between five and 70 kilometres thick, constituting less than one per cent of Earth's volume) of our planet's lithosphere (composed of the crust and the upper portion of the mantle).
The Earth tide encompasses our entire planet, unhindered by the landmasses on the Earth's surface; it is on such a global scale that the rigidity of rock is irrelevant.
The alignment of the sun and moon at the new and full lunar phases results in the most influence, while the first- and last-quarter lunar phases have minimal influence. It is also not noticeable to us humans walking on the Earth's surface, due to the fact that the overall amplitude or lift of the ground from the Earth tide (about 56 centimetres at the equator, less elsewhere) is negligible, as it's spread over such a wide area of our planet's surface.
However, this flexing upward and downward movement (referred to as "displacement") is routinely monitored and recorded by numerous scientific instruments located around the world.
While we may not be aware of this movement beneath our feet as we go about our daily lives, the flexing of the Earth's crust does, nonetheless, impact us in a number of ways. The Global Positioning System (GPS) that we have all come to rely on to position us and our technologies on the Earth's surface; satellite laser-ranging; very-long-baseline (VLB) interferometry (used in radio astronomy); and precise astronomical angular measurements are all dependent on the Earth's rate of rotation, which, in turn, is directly influenced by the Earth tides.
Earth tides can also have an effect on volcanic eruptions and seismic (earthquake) activity around the world, although it is only one of a number of gravitational influences, along with geomorphological processes (such as tectonic plate movements), that cause such activity.
It is, however, just one more demonstration of how our planet, and our lives upon it, are impacted by the celestial objects above us, distant though they may be.
THIS WEEK’S SKY
Mercury and Mars are too close to the sun to be observed this coming week.
Venus (magnitude -4.0, seen in Virgo — the Virgin) becomes visible around 8:15 p.m. ADT (8:45 p.m. NDT), eight degrees above the western horizon as dusk gives way to darkness, before sinking towards the horizon, and setting around 9:10 p.m. ADT (9:40 p.m. NDT).
Saturn (magnitude +0.3, visible in Capricornus — the Sea Goat) becomes visible 14 degrees above the southeast horizon by about 8:30 p.m. ADT (9:00 p.m. NDT), reaching a height of 24 degrees above the southern horizon by 11:15 p.m. ADT (11:45 p.m. NDT), and remaining visible until about 2:40 a.m. ADT (3:10 a.m. NDT), when it drops below 10 degrees above the southwest horizon.
Jupiter (magnitude -2.9, look for it in Capricornus) appears around 8:25 p.m. ADT (8:55 p.m. NDT), eight degrees above the southeast horizon, reaching its highest point of 29 degrees above the southern horizon around 12:35 a.m. ADT (1:05 a.m. NDT), and becoming unobservable by about 4:45 a.m. ADT (5:15 NDT), when it sinks below seven degrees above the southwest horizon.
Until next week, clear skies.
EVENTS
• Aug. 30 — Last Quarter Moon
• Sept. 5 — Venus is 1.7 degrees north of Spica (in Virgo), low in west-southwest sky after sunset