Astronomy - USA 2021-04)

14 ASTRONOMY • APRIL 2021

STRANGE UNIVERSE

In New York City, it’s prestigious to say that

you live on the East Side. Conversely, when the

film We st Sid e Stor y c a me out , e ver yone k new

that title referred to rough neighborhoods with street

gangs. Directions matter, at least here on Earth.

When we explore the universe, the east-west distinc-

tion ranges from critical to irrelevant. A huge, near-

light-speed jet races out of the west side of the galaxy

M87, but few observers know or care that it’s heading

toward Leo.

These directions can also get strangely

confusing. When Venus is a morning star, it

can only be seen in the east. Yet at such times,

Venus is listed as having a western elonga-

tion. Same with Mercury. On March 6, that

charbroiled world reaches its greatest western

elongation of 27°, its best of the year. But if

you look west, you won’t see it. You must

instead face east just before dawn.

Now is the best time to be pondering this,

since the equinox highlights these cardinal directions.

On Saturday, March 20, the Sun rises precisely due east

and sets exactly in the west. Those directions have their

singular origin in our planet’s spin. But for the Sun to

rise at the exact eastern cardinal spot on the horizon,

our world cannot be tilted toward or away from our star,

which rarely happens. Instead, it must stand perfectly

sideways to the Sun. This occurs only at the equinox.

So, orientation matters. As an interesting aside — to

be oriented means you correctly know which way you’re

facing. The word shares some meaning with the out-

dated term Orient, meaning — for those in Europe

— the East. Thus, originally, if you knew which way east

was, you were oriented.

As on Earth, orientation matters in astronomy as well.

Directions from binary stars to their dimmer compan-

ions routinely allude to the east or west. And here, we’ve

all occasionally been thrown for a loop since our mir-

rors, lenses, and star diagonals f lip or reverse the images.

Many observers have discovered that a quick way to get

reoriented is to turn off the drive motor. When the object

starts drifting, it is reliably toward the west.

And this would be good enough, except the Celestial

Police — the International Astronomical Union (IAU)

— issued an astonishing decree in 1961. Until then,

when the Moon hovered on the meridian at its highest

point each night, you knew its right side was its western

side, since that limb faced your western horizon.

Indeed, as Earth rotated, the Moon’s leading edge across

the sky was its right, or western, limb.

The trailing limb, the one closest to your eastern hori-

zon, was, of course, its eastern side. So, when the German

astronomer Julius Franz discovered a major new lunar

mare in 1906 right smack on its leftmost edge, he named

it Mare Orientale — the Eastern Sea. This dramatic

feature comprises a concentric set of circular ridges, like

a bull’s-eye target. It became more widely known after

British popularizer Patrick Moore claimed in a bewilder-

ing entry in the 1976 edition of his book Guide to the

Moon that he had discovered and named it. (Thirty years

later, Moore conceded Franz’s priority.) What’s relevant

is that both authors affirmed the mare’s eastern locale.

But with President John F. Kennedy’s promise that

the U.S. would soon send astronauts to the Moon, a

strange directional issue surfaced. On ter-

restrial maps, west is always to the left. If

people would now explore the Moon, find

new features, and make new charts, shouldn’t

those maps match earthly ones?

“Done!” proclaimed the IAU in 1961.

Overnight, the right side of the lunar limb was

officially designated its eastern limb, even

though it was on the Moon’s west side. Which

br i ngs u s back to Ma re Or ient a le , t hat e a ster n

sea. Yep, you guessed it — they kept the name,

even though it was now located smack on the Moon’s

newly minted western edge. And this month around the

equ i nox, a s t he ter m i nator s we eps over t he wa x i ng Moon,

although it nightly progresses eastward as measured by

any compass, GPS, or the sky’s universal directions, it’s

officially moving west along the Moon’s face.

When Rudyard Kipling said of east and west that

“never the twain shall meet,” he had no idea what was

coming a mere quarter-century after his death.

Celestial directions bring on an added layer of confusion.

East meets west?

Mare Orientale, the

Eastern Sea, is

located on the left

side of the Moon, as

viewed from Earth.

Following a 1961

ruling by the IAU, this

is the Moon’s western

edge. NASA’S SCIENTIFIC

VISUALIZATION STUDIO; NASA/

GSFC/ARIZONA STATE UNIV./LUNAR

RECONNAISSANCE ORBITER

Originally,

if you knew

which way

east was,

you were

oriented.

BROWSE THE “STRANGE UNIVERSE” ARCHIVE

AT http://www.Astronomy.com/Berman

BY BOB BERMAN

Bob’s newest book,

Earth-Shattering

(Little, Brown and

Company, 2019),

explores the greatest

cataclysms that have

shaken the universe.