Le Trung Hieu

Only in Photoshop does Mars appear as large as a full Moon

Mars in August 2003 during a 60,000-year record close approach. Even then, the planet resembled a bright star, not a full Moon. Photo credit: John Nemy & Carol Legate of Whistler, B.C.

For the sixth year in a row, a message about the Red Planet is infecting worldwide email boxes. It instructs readers to go outside after dark on August 27th and behold the sky. "Mars will look as large as the full moon," it says. "No one alive today will ever see this again."

Don't believe it.

Here's what will really happen if you go outside after dark on August 27th. Nothing. Mars won't be there. On that date, the red planet will be nearly 250 million km away from Earth and completely absent from the evening sky.

The Mars Hoax got its start in 2003 when Earth and Mars really did have a close encounter. On Aug. 27th of that year, Mars was only 56 million km away, a 60,000-year record for martian close approaches to Earth. Someone sent an email alerting friends to the event. The message contained some misunderstandings and omissions—but what email doesn't? A piece of advanced technology called the "forward button" did the rest.

Tolerant readers may say that the Mars Hoax is not really a hoax, because it is not an intentional trick. The composer probably believed everything he or she wrote in the message. If that's true, a better name might be the "Mars Misunderstanding" or maybe the "Confusing-Email-About-Mars-You-Should-Delete-and-Not-Forward-to-Anyone-Except-Your-In-Laws."

Another aspect of the Mars Hoax: It says "Mars will look as large as the full Moon if you magnify it 75x using a backyard telescope." The italicized text is usually omitted from verbal and written summaries of the Hoax. (For example, see the beginning of this story.) Does this fine print make the Mars Hoax true? After all, if you magnify the tiny disk of Mars 75x, it does subtend an angle about the same as the Moon.

No. Even with magnification, Mars does not look the same as a full Moon.

This has more to do with the mysterious inner workings of the human brain than cold, hard physics. Looking at Mars magnified 75x through a slender black tube (the eyepiece of a telescope) and looking at the full Moon shining unfettered in the open sky are two very different experiences.

A good reference is the Moon Illusion. Moons on the horizon look huge; Moons directly overhead look smaller. In both cases, it is the same Moon, but the human mind perceives the size of the Moon differently depending on its surroundings.

Likewise, your perception of Mars is affected by the planet's surroundings. Locate the planet at the end of a little dark tunnel, and it is going to look tiny regardless of magnification.

Bummer!

To see Mars as big as a full Moon, you'll need a rocketship, and that may take some time. Meanwhile, beware the Mars Hoax.

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Earth to Have Closest Encounter With Jupiter until 2022

Monday, September 27, 2010 1:51:50 PM

Jupiter's Great Red Spot.
Image Credit: NASA.

Been outside at midnight lately? There's something you really need to see. Jupiter is approaching Earth for the closest encounter between the two planets in more than a decade--and it is dazzling.

The night of closest approach is Sept. 20-21st. This is also called "the night of opposition" because Jupiter will be opposite the sun, rising at sunset and soaring overhead at midnight. Among all denizens of the midnight sky, only the moon itself will be brighter.

Earth-Jupiter encounters happen every 13 months when the Earth laps Jupiter in their race around the sun. But because Earth and Jupiter do not orbit the sun in perfect circles, they are not always the same distance apart when Earth passes by. On Sept. 20th, Jupiter will be as much as 75 million km closer than previous encounters and will not be this close again until 2022.

The view through a telescope is excellent. Because Jupiter is so close, the planet's disk can be seen in rare detail--and there is a lot to see. For instance, the Great Red Spot, a cyclone twice as wide as Earth, is bumping up against another storm called "Red Spot Jr." The apparition of two planet-sized tempests grinding against one another must be seen to be believed.

Also, Jupiter's trademark South Equatorial Belt (SEB) recently vanished, possibly submerging itself beneath high clouds. Researchers say it could reappear at any moment. The dramatic resurgence would be accompanied by a globe-straddling profusion of spots and cloudy swirls, clearly visible in backyard telescopes.

And what was that flash? Amateur astronomers have recently reported a surprising number of fireballs in Jupiter's atmosphere. Apparently, many small asteroids or comet fragments are hitting the giant planet and exploding among the clouds. Researchers who have studied these events say visible flashes could be occurring as often as a few times a month.

Finally, we mustn't forget the moons of Jupiter because they are also having a close encounter with Earth. These are planet-sized worlds with active volcanoes (Io), possible underground oceans (Europa), vast fields of craters (Callisto), and mysterious global grooves (Ganymede). When Galileo discovered the moons 400 years ago, they were no more than pinpricks of light in his primitive spy glass. Big, modern amateur telescopes reveal actual planetary disks with colorful markings.

It makes you wonder, what would Galileo think?

Answer: "I'm getting up at midnight!"

Dr. Tony Phillips
Science@NASA

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Andromeda

Sunday, September 26, 2010 9:07:24 AM

Thiên hà Andromeda, nổi bật lên các hạt bụi lốm đốm từ những cánh tay xoắn ốc của nó. Bức ảnh này được chụp bằng kính thiên văn hồng ngoại WISE (NASA's Wide-field Infrared Survey Explorer), ánh sáng có bước sóng 12 micromet đã được mã hóa thành màu cam và 22 micromet là màu đỏ.

Các hạt bụi sáng, được nung nóng bởi những ngôi sao mới sinh ra , vươn mình theo hình xoắn ốc tới trung tâm của thiên hà.

Thiên hà Andromeda, còn được gọi là M31, cách chúng ta 2.5 triệu năm ánh sáng. Nó là thiên hà láng giêng` gần chúng ta nhất.

Image credit: NASA/JPL-Caltech/UCLA

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Chòm Thất Tinh ( The Pleiades )

Saturday, September 25, 2010 1:01:08 PM

Bức ảnh cụm sao Thất tinh ( Pleiades ) nổi tiếng này được chụp bằng kính thiên văn hồng ngoại WISE của NASA.

Pleiades được các nhà thiên văn gọi là cụm sao mở, nghĩa là gồm các ngôi sao liên kết với nhau .Và sau đó khoàng vài trăm triệu năm, chúng sẽ tách rời . Cụm sao này nồi bật trên bầu trời mùa đông trong chòm sao Kim ngưu ( Taurus ), khi nhìn từ bán cầu bắc.

Thần thoại Hy Lạp gọi cụm sao này là 7 chị em (Seven Sister ) . Ngoài ra, nó còn có nhiều cái tên khác như Parveen ở Persian, Tianquiztli ở Aztec, Subaru ở Nhật Bản.

Trong bức ảnh chụp hồng ngoại của WISE , cụm sao này được bao quanh bởi các đám mây bụi khổng lồ. Khi đám mây này lần đầu tiên được quan sát, nó được cho là phần vật chất còn sót lại từ việc hình thành các cụm sao. Ánh sáng từ các cụm sao có lẽ đã đi xuyên qua đám mây được nhìn thấy, nung nóng nó lên và làm nó phát sáng.

Ở khoảng cách 436 triệu năm ánh sáng từ Trái đất, Thất tinh được coi là một trong những cụm sao gần Trái đất nhất. Nó đóng vai trò quan trọng trong việc xác định khoảng cách tới các vật thể thiên văn xa xôi.

Kính thiên văn WISE được trang bị 4 máy dò hồng ngoại. Trong bức ảnh, màu xanh và màu lục lam tượng trưng cho ánh sáng hồng ngoại ở bước sóng từ 3.4 đến 4.6 micromet, hầu hết đến từ các ngôi sao. Màu xanh và đỏ tượng trưng cho ánh sánh từ 12 đến 22 micromet, phản chiếu từ các hạt bụi.

Ảnh: NASA/JPL-Caltech/UCLA

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