Tag: gravity

What’s GRAV-D?

The pilot and co-pilot of a Navy-owned King Air aircraft prepare for takeoff on a data collection flight with GRAV-D instruments (fore and mid-ground) and a GRAV-D operator as their payload.

The pilot and co-pilot of a Navy-owned King Air plane put together for takeoff on an information assortment flight with GRAV-D devices (fore and mid-ground) and a GRAV-D operator as their payload.


So, what does that imply? First, to outline a few phrases: Gravity is the power that causes objects to fall to, and keep on, the bottom. Datums are the idea for all geodetic survey work, and vertical datums are used to measure land elevations and water depths.

Map of variations in Earth's gravity as measured by the GRAV-D airborne instruments over Lake Michigan and surrounding states.

Map of variations in Earth’s gravity as measured by the GRAV-D airborne devices over Lake Michigan and surrounding states.

The vertical datum is set, or “realized,” by a group of particular factors on the Earth with identified heights. In 2007, NOAA’s Nationwide Geodetic Survey started a 15-year undertaking to redefine the datum based mostly on gravity measurements.

GRAV-D measures and displays Earth’s gravity area to help the geoid—a mannequin of roughly international imply sea stage used to measure exact floor elevations—in order that it might function the “zero reference floor” for all heights within the nation. Correct heights are essential to many scientific endeavors, however are significantly vital for safeguarding low-lying coastal ecosystems.

Variations within the dimension, form, and mass distribution of the Earth trigger slight variations within the “energy” of gravity’s pull. GRAV-D measures gravity from an plane and people information are mixed with satellite tv for pc and floor gravity measurements to create the geoid. When the undertaking is accomplished in 2022, the geoid will probably be utilized to satellite tv for pc place measurements like GPS to acquire extraordinarily correct heights (to inside 2 cm the place doable).

It’s estimated that GRAV-D will present $4.8 billion in socioeconomic advantages to the US by improved floodplain mapping, coastal useful resource administration, development, agriculture, and emergency evacuation planning.

Are tides larger when the moon is instantly overhead?

Earth viewed from the surface of the moon

This NASA picture from the Apollo 8 mission exhibits the Earth seen over the horizon of the moon. Whereas the moon and solar trigger tides on our planet, the gravitational pull of those celestial our bodies doesn’t dictate when excessive or low tides happen.


Tides originate within the ocean and progress towards the coastlines, the place they seem because the common rise and fall of the ocean floor. Because of Sir Isaac Newton’s 1687 discovery, we all know that tides are very long-period waves that transfer by the ocean in response to forces exerted by the moon and solar. Nevertheless, these gravitational forces don’t management when excessive or low tide occasions happen. Different forces, extra regional than the moon or solar, management the tides. Lots of them need to do with the geography and form of the Earth.

The form of our planet has loads to do with variations in gravitational pull at numerous places. If Earth was a real sphere lined by an ocean of fixed depth, then it might be true {that a} excessive tide occasion would happen on the location with the moon overhead. The tidal “bulge” would transfer across the Earth with the moon, however this isn’t the case with our planet. The Earth just isn’t a real sphere, however bulges barely on the Equator. Additionally it is dotted with massive land lots (continents). Areas the place the Earth’s floor is larger, akin to mountains, have a stronger gravitational power than do locations the place the floor is decrease, akin to a valley or cavern. On the similar time, the depth of the world ocean varies vastly. All of those components play into the peak of the tides.

One other factor to think about when enthusiastic about tides is the dimension of time. There are lags in time when an observer experiences excessive or low tide, and the lag varies from one location to a different. For instance, the rising slope of the ocean ground approaching the U.S. East (often known as the continental rise, continental slope, and continental shelf) slows down the tidal “bulge” or “wave” of water. The lag may be hours and even near a complete day in some elements of the world. Consider the wave or bulge that you simply see if you throw a rock right into a pond. The wave travels outward in a hoop. If there’s a tree or different obstruction within the pond, the ring of waves would journey and wrap round it. The identical holds true for the tides.

NOAA’s Heart for Operational Oceanographic Merchandise and Providers makes use of superior acoustic and digital tools to measure and monitor the tides alongside our coasts. NOAA has water remark stations in additional than 3,000 places across the U.S. to look at water ranges and generate tidal predictions.