Earth will have its last to­tal so­lar eclipse in about 600 mil­lion years

Weekend Mirror - - CHILDREN’S CORNER -

Dur­ing

the to­tal so­lar eclipse on Aug. 21, sky­watch­ers will di­rect most of their at­ten­tion to the sun, but don’t for­get about the moon: Its slow progress away from Earth means these ce­les­tial events won’t keep hap­pen­ing for­ever.

Next month’s to­tal so­lar eclipse will sweep across the con­ti­nen­tal U.S. from Ore­gon to South Carolina along a stretch of land about 70 miles (113 kilo­me­ters) wide. A to­tal so­lar eclipse oc­curs only when the disk of the moon passes be­tween the Earth and the sun, briefly block­ing the sun’s bright light and cast­ing a long shadow on the planet.

“A to­tal eclipse is a dance with three part­ners: the moon, the sun and Earth,” Richard Von­drak, a lu­nar sci­en­tist at NASA’s God­dard Space Flight Cen­ter in Mary­land, said in a state­ment. “It can only hap­pen when there is an ex­quis­ite align­ment of the moon and the sun in our sky.”

To­tal so­lar eclipses oc­cur be­cause the moon and the sun have the same ap­par­ent size in Earth’s sky — the sun is about 400 times wider than the moon, but the moon is about 400 times closer.

But the moon is slowly mov­ing away from Earth by about 1-1/2 inches (4 cen­time­ters) per year, ac­cord­ing to the NASA state­ment. As a re­sult, to­tal so­lar eclipses will cease to ex­ist in the very dis­tant fu­ture, be­cause the ap­par­ent size of the moon in Earth’s sky will be too small to cover the sun com­pletely.

“Over time, the num­ber and fre­quency of to­tal so­lar eclipses will de­crease,” Von­drak said in the state­ment. “About 600 mil­lion years from now, Earth will ex­pe­ri­ence the beauty and drama of a to­tal so­lar eclipse for the last time.”

For now, a to­tal so­lar eclipse is vis­i­ble from some­where on the Earth’s sur­face about once ev­ery 18 months, on av­er­age. How­ever, see­ing a to­tal so­lar eclipse from a spe­cific lo­ca­tion is rare, be­cause the moon’s in­ner shadow is rel­a­tively small, which lim­its the to­tal area from which the to­tal eclipse is vis­i­ble, ac­cord­ing to a video from NASA about the moon’s role in a so­lar eclipse.

“You have to be on the sunny side of the planet, and you have to be in the path of the moon’s shadow,” NASA of­fi­cials said in the video. “So, if you find your area in the path of to­tal­ity one year, you’ve hit the jack­pot, be­cause on av­er­age, that same spot on Earth will only get to see a so­lar eclipse ev­ery 375 years.”

But note that par­tial so­lar eclipses, in which the moon ob­scures only part of the sun, are vis­i­ble across a much larger area. The two parts of the moon’s shadow, the um­bra and penum­bra, de­ter- mine which kind of eclipse an ob­server sees on Earth. The moon’s um­bra, or the dark in­ner shadow, is the part of the moon’s shadow where the en­tire sun is blocked by the moon. The penum­bra is where only part of the sun’s disk is ob­scured.

Most eclipse maps high­light­ing the path of to­tal­ity show a dark cir­cle that rep­re­sents the um­bra. How­ever, the “true shape of the um­bra is more like an ir­reg­u­lar poly­gon with slightly curved edges,” ac­cord­ing to the video. Fea­tures on the sur­face of the moon de­ter­mine the shape of the um­bra.

Us­ing data from NASA’s Lu­nar Re­con­nais­sance Or­biter (LRO), NASA sci­en­tists were able to map the lu­nar sur­face in un­prece­dented de­tail, show­ing the moun­tains and val­leys that af­fect the pass­ing sun­light and sub­se­quent shape of the moon’s shadow dur­ing a to­tal so­lar eclipse, ac­cord­ing to the video.

These to­po­graphic maps, along with Earth el­e­va­tion data, al­low sci­en­tists to de­ter- mine the ex­act ar­eas on Earth that fall in the path of to­tal­ity for the Aug. 21 so­lar eclipse.

“With this new vi­su­al­iza­tion, we can rep­re­sent the um­bral shadow with more ac­cu­racy by ac­count­ing for the in­flu­ence of el­e­va­tion at dif­fer­ent points on Earth, as well as the way light rays stream through lu­nar val­leys along the moon’s ragged edge,” Ernie Wright, a NASA vi­su­al­izer at God­dard, said in the state­ment.

The LRO data also helps sci­en­tists bet­ter pre­dict when and where sky­watch­ers will see “Baily’s beads,” the ir­reg­u­lar dots of light vis­i­ble around the edge of the moon dur­ing a to­tal so­lar eclipse. This phe­nom­e­non is caused by the last rays of sun­light stream­ing through the moon’s rugged moun­tain val­leys, and it oc­curs for just a few sec­onds be­fore and after to­tal­ity.

So when you look up for the so­lar eclipse on Aug. 21 — whether you are see­ing a to­tal or par­tial eclipse — be sure to ad­mire the moon as well as the sun.

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