Earth MoonDirection of
Earth’s rotationHigh tide High tideLow tideLow tideDirection of
Moon’s motion44 ASTRONOMY • JANUARY 2018A: Tides occur because of the
uneven pull of the Moon’s
gravity on different parts of
Earth. Portions closer to the
Moon are pulled more strongly
than those farther away. As the
Moon pulls on the portion of
the planet nearest its location,
the water deforms and bulges
toward the Moon more easily
than the seafloor beneath. On
the far side of Earth, the water
is “left behind” as the rest of
the Earth feels a stronger
attraction to the Moon, causingthe water to pile up and bulge
away from the planet. Earth’s
rotation causes most locations
to experience these two bulges
each day, approximately 12
hours apart.
The Sun also inf luences
tides on Earth, though its effect
is slightly less than half as
strong as the Moon, due to its
greater distance. During the
Full and New Moon phases
when Earth, the Moon, and the
Sun are aligned, these gravita-
tional interactions reinforceAstronomy’s experts from around the globe answer your cosmic questions.CELESTIAL
MOTION
each other. This gives us the
highest high tides and lowest
low tides (called spring tides).
During the First and Last
Quarter Moon, the tidal forces
from the Sun and Moon are
working in different directions,
leading to smaller tidal bulges
(neap tides).
Tides are even more com-
plex than this; not all places on
Earth have two equal high and
low tides per day (called semi-
diurnal tides). Some places,
such as the Gulf of Mexico,
only have one high and one low
tide per day because the sur-
rounding landmasses prevent
the free f low of water through-
out the globe. As a result, more
complex patterns occur in par-
ticular regions.
April Russell
Visiting Professor, Siena College,
Loudonville, New YorkQ: WHAT ARE THE MOTIONS
OF STARS RELATIVE TO
EACH OTHER IN A GLOBU-
LAR OR OPEN CLUSTER?
DOES THE CLUSTER MOVE
AS A UNIT OVER TIME?
George Haskins
Auburn, WashingtonA: Globular clusters are com-
pact groups of up to a million
or so stars held together by
their mutual gravitationalattraction, with a nearly spheri-
cal distribution and high den-
sity in the center. Our own
galaxy has approximately 170
globular clusters separated into
two subsystems associated with
the galactic disk and halo. The
movement of halo star clusters
traces the galactic gravitational
field at large scales, and their
spatial distribution provides
stringent constraints for mod-
els of dark matter distribution
in the outer parts of the galaxy.
In a globular cluster, star
motions are determined by the
sum of the mass of all stars
within the cluster. The cluster’s
internal dynamics are also
affected by its “relaxation
time,” which is the time it takes
for random encounters
between stars to erase informa-
tion about their initial orienta-
tion. For globular clusters, the
average relaxation time is
shorter than their age, so it can
be argued that they are close to
a relaxed state, like air mol-
ecules at room temperature.
This is the physical reason why
the orbits of member stars do
not have a preferential orienta-
tion (i.e., their distribution is
“isotropic”) around the center
of mass of the cluster itself.
This is even more pronounced
in open clusters, which are
typically smaller systems of
only a few thousand stars.ASKASTR0
Q: I SAW A PROGRAM THAT SHOWED
THE MOON REVOLVING AROUND EARTH,
CAUSING THE OCEANS TO SWELL ON
BOTH SIDES OF THE PLANET. SHOULDN’T
THE OCEAN OPPOSITE THE MOON BE
SHALLOWER THAN THE SIDE FACING IT?
Michael Gamble, New Berlin, IllinoisThe globular cluster Messier 68 is 33,000 light-years from Earth and spans
about 100 light-years. At least 2,000 bright stars are easily visible, but this
cluster likely contains several tens of thousands more.The ocean nearest the Moon bulges outward in a high tide because of
increased gravitational attraction. On the other side of Earth, the ocean
bulges outward as the rest of the planet is pulled toward the Moon. Low
tide occurs at the two points on Earth between the bulges. ASTRONOMY: ROEN KELLYESA/HUBBLE AND NASA