Cooler water dissolves seashells faster. The Gulf Stream flowing north along eastern United States does indeed make Atlantic coastal waters warmer than the Pacific along the U.S. western shore. Furthermore, chalky substances like seashells dissolve faster in cooler waters. "Thus, even if you start with the same density of seashells on eastern and western beaches, you would end up with lower densities on western beaches than on eastern ones because the shells would disappear faster in western colder waters," says Pineda.

Bivalves’ favor eastern beaches. Most seashells are bivalves (two hinged shells like a clam’s) and these creatures like to live in soft sediment. So, they favor sandy beaches over rocky coastlines.

Long, trailing tectonic ledges form the eastern shoreline as the Eurasian Plate pulls away from the North American Plate and the Atlantic Ocean widens. The broad ledges drain the gently sloping land into many rivers that wash sediment out to sea. Thus, sandy beaches predominate along the east coast—the kind that bivalves love.

On the other hand, the Pacific Plate colliding with the North American Plate forms an abrupt western coast. Consequently, the U.S. western coastline has a smaller drainage area and less sediment to form beaches.

That’s why more bivalves—the most abundant seashell—exist along eastern than western shores.

But, of course, there are exceptions. The shores of the Gulf of California have "the highest densities of seashells I have ever seen," says Pineda.

Woods Hole Oceanographic Institution: Dive and discover

[April Holladay] Mass extinction time line

A: About five mass extinctions have devastated life over its 4-billion history. The first and one of the most devastating extinctions was caused by an ice age 450 million years ago. It coincided with the greatest extent of ice-age glaciers. Many animal and plant genera (whole groups of species) died and diversity withered. Then, about 350 million years ago, both plant and animal species died slowly on a massive scale. The extinction event coincided with and perhaps was caused by seas that fell to low levels and volcanoes that belched noxious fumes.

The next two mass extinctions happened fairly close together: 250 and 200 million years ago. Ninety-six percent of the world’s species perished 250 million years ago in the greatest mass extinction in Earth’s history. Falling sea levels, rising temperatures, and volcanic fumes may have been responsible. The next, 50 million years later, wiped out 65 % of marine animals and 90 % of land plant species. We’ve identified two likely causes: temperature and meteor impact. Temperatures rose dramatically, apparently due to a greenhouse effect. A meteor fell, smashing Quebec, Canada and blasting one of the largest known craters. The debris clouded the atmosphere and blocked sunlight reaching Earth.

The last and most famous mass extinction occurred 65 million years ago. It devastated dinosaurs—not to mention: flying reptiles, swimming reptiles, belemnites, ammonites, specialized bivalves of the oceans, and many fish. An asteroid or comet may have crashed explosively off the Yucatan coast and killed the reptiles.

Mass extinctions give other species a chance to fill old niches. Were it not for the last one, we might all be reptiles instead of naked apes.

Q: How do we know what time it really is? I know we can measure time intervals very accurately, but what do we use to set the time? —Eric B.

[Commander John Bortniak NOAA] Our timepiece: the Sun

A: Setting time is as arbitrary as setting zero on a circle. We have different approaches. Let’s discuss time in a frame of reference that’s moving at a constant speed: Earth.

"[Time] presents to us a sequence of changes," says Guy Ottewell in his book, The Astronomical Companion. We use the periodic transitions to set time. The change that dominates our lives is daylight: coming and going. Sundials measure Sun time. We set them to the time we measure most reliably—high noon, when the Sun is nearly overhead and due south (north for folks in the Southern Hemisphere). This illustrates a point nicely: time is direction (at noon the sunward direction is south). It’s called "true", "Sun", or "apparent" time. However, Sun time is inconvenient since it varies over Earth and through the year.

"From 1925 on, the GMT day was changed to run from midnight to midnight," says Robert Massey, astronomer at the Royal Observatory at Greenwich. However, GMT isn’t really defined now. The time function moved years ago to the National Physical Laboratory in southwest London. GMT is related to Coordinated Universal Time (UTC) in some contexts and to Universal Time (UT) in others. UTC is a more accurate atomic timescale; UT is based on Earth’s rotation. Timekeepers correct UT to UTC time through the use of leap seconds that compensate for the slowing of the Earth’s rotation over the years, says Massey.

Astronomers give astronomical event times in UT. For example, the closest conjunction of planets this year occurs on March 28 at 13h UT, which is 13 - 5 = 8 a.m. EST (Eastern Standard Time).

EST lags GMT by five hours because the east U.S. coastal states are west of London by 75 longitudinal degrees. Earth spins from west to east at a rate of 15 degrees per hour. So it takes five hours for New Yorkers to see the Sun (nearly) overhead after it's noon in Greenwich/London.

By the way, not all countries, even today, use standard time. Saudi Arabia still uses Sun time and Saudis know it’s noon when the Sun is due south and passes nearly overhead.

U.S. Naval Observatory: Time FAQ

(Answered March 28, 2003)