Omicron Ceti : Mira the Wonderful Star

Gerhana Bulan terjadi saat Matahari-Bumi-Bulan berada dalam satu garis lurus. Keadaan itu menyebabkan sinar Matahari yang seharusnya menyinari Bulan tersabotase oleh Bumi. Dengan kata lain, Bumi menghalangi Matahari untuk menyinari Bulan. Gerhana Bulan Total sendiri terjadi ketika seluruh penampang Bulan tertutupi oleh bayangan Bumi, lebih tepatnya oleh bayangan umbra Bumi
.

Umbra dan Penumbra. Sumber: Sky & Telescope

Umbra adalah bayangan pusat atau inti yang gelap. Selain umbra ada pula Penumbra yang merupakan bayangan kabur di sekitar umbra.

Meskipun Bulan memasuki bayangan umbra yang gelap, Bulan akan tetap terlihat karena atmosfer Bumi merefraksikan sinar Matahari. Itulah penyebab Bulan nampak suram berwarna merah kehitaman saat gerhana berlangsung.

Setelah disuguhkan keindahan gerhana bulan total 16 Juni 2011 silam, diperkirakan keindahan itu akan berulang kembali pada 10 Desember 2011 mendatang. Saat itu Bulan sedang berada di rasi Taurus di langit sebelah timur.

Pada gerhana Bulan kali ini, lintasan orbit Bulan akan melewati bayangan umbra Bumi bagian selatan. Walaupun gerhana Bulan total ini bukanlah gerhana sentral (Bulan melewati pusat umbra Bumi), tetapi gerhana ini bakal berlangsung cukup lama, fase gerhana bulan total (U2 hingga U3) selama 51 menit 8 detik, fase kontak umbra (U1 hingga U4) selama 3 jam 32 menit 15 detik, dan fase kontak penumbra (P1 hingga P2) 5 jam 56 menit 21 detik.

Sistematik Gerhana Bulan Total 10 Desember 2011. Sumber: Fred Espenak / NASA

Berikut rincian lengkap mengenai sistematik dan waktu tiap fase terjadinya gerhana bulan total 10 Desember 2011:

• Penumbra 1 (P1) pukul 18:33 WIB; 19:33 WITA; 20:33 WIT 

Bulan mulai masuk ke penumbra, Bulan meredup - gerhana dimulai.

• Umbra 1 (U1) pukul 19:45 WIB; 20:45 WITA; 21:45 WIT

Bulan mulai memasuki umbra, bentuk Bulan berubah sabit - gerhana bulan sebagian dimulai.

• Umbra 2 (U2) pukul 21:06 WIB; 22:06 WITA; 23:06 WIT

Seluruh bagian Bulan memasuki umbra, Bulan melegam - gerhana bulan total dimulai. 

• Puncak Gerhana Bulan Total pukul 21:31 WIB; 22:31 WITA; 23:31 WIT

Puncak gerhana bulan total, puncak bulan fase purnama. 

• Umbra 3 (U3) pukul 21:57 WIB; 22:57 WITA; 23:57 WIT

Bulan mulai keluar dari umbra, Bulan kembali nampak - gerhana bulan total selesai.

• Umbra 4 (U4) pukul 23:17 WIB; 00:17 WITA; 01:17 WIT

Seluruh bagian Bulan keluar dari umbra, Bulan kembali purnama - gerhana bulan sebagian selesai. 

• Penumbra 2 (P2) pukul 00:29 WIB; 01:29 WITA; 02;29 WIT

Bulan keluar dari penumbra, Bulan kembali terang - gerhana bulan selesai.

Catatan: Waktu sipil yang digunakan (WIB) merujuk pada bujur 105° BT. Gunakan waktu lokal untuk jadwal fase gerhana yang lebih akurat.

Negara-negara beruntung yang dapat menyaksikan keseluruhan fase dari gerhana bulan total kali ini adalah Rusia, Mongolia, China, Nepal, Bhutan, Bangladesh, Myanmar, Thailand, Laos, Kamboja, Vietnam, Malaysia, Singapura, Filipina, Hongkong, Taiwan, Korea Utara, Korea Selatan, Jepang, Indonesia, Papua Nugini, dan Autralia. Itu artinya seluruh bagian Indonesia -baik barat, tengah, maupun timur- dapat mengamati fase Gerhana Bulan Total secara lengkap.

Selain negara-negara diatas, sebagaian besar negara di Eropa, Afrika, dan Amerika Utara pun dapat mengamati gerhana ini. Namun, tidak semua fase gerhana dapat diamati dari sana.

Peta daerah yang dilewati GBT 10 Desember 2011. Sumber: Wikipedia

Gerhana kali ini termasuk dalam siklus saros 135 yang memiliki 71 seri gerhana dimana nanti adalah seri ke 23-nya. Setiap seri gerhana dalam suatu siklus saros akan memiliki geometri (Tata letak atau koordinat Matahari, Bulan dan jarak keduanya dari Bumi) gerhana yang sama. Perlu diketahui antar seri gerhana dalam satu siklus saros berselang 18 tahun 11 hari 8 jam. Dengan begitu 18 tahun 11 hari 8 jam kemudian (Terhitung pada 20 Desember 2029) akan terjadi seri gerhana ke 24 dengan geometri serupa.

Seperti gerhana bulan total sebelumnya, pengamatan kali ini dapat dilakukan dengan atau tanpa alat bantu. Pengamatan pun dianjurkan di tempat minim polusi cahaya dengan medan pandang langsung mengarah ke Bulan tanpa tertutupi apapun.

Gimana? Seru kan pasti ngamat gerhana bulan pas malem minggu? 

Data Teknis:

Matahari
Asensiorekta: 17h08m35s
Deklinasi: -22°53'44"
Magnitudo: -26.78
Diameter sudut: 32'29"
Jarak dari Bumi: 0.98 AU

Bulan
Asensiorekta: 5h09m58s
Deklinasi: +22°55'38"
Magnitudo: 1.1
Diameter sudut: 30'25"
Jarak dari Bumi:0.0026 AU

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The Magic Moneybag

A Korean Folktale

Long, long ago there was a young couple who lived in a small thatched hut in a gully. They were so poor that every day they had to cut two bundles of firewood and carry them to market on their backs.
One day, the young couple came back from the mountain carrying the firewood. They put one bundle in the courtyard and planned to sell it at the market the next day to buy rice. The other bundle they kept in the kitchen for their own use. When they woke up the following morning, the bundle in the courtyard had mysteriously disappeared. There was nothing to do but to sell the bundle which they had kept for themselves.
That same day, they cut another two bundles of firewood as usual. They put one bundle in the courtyard for market and kept the other bundle for their own use. But the following morning, the bundle in the courtyard had vanished again. The same thing happened on the third and fourth day as well, and the husband began to think there was something strange going on.
On the fifth day, he made a hollow in the bundle of firewood in the courtyard and hid himself inside it. From the outside it looked just the same as before. At midnight an enormous rope descended from the sky, attached itself to the bundle and lifted it up into the sky, with the woodcutter still inside it.
On his arrival in heaven, he saw a kindly looking, white-haired old man coming in his direction. The old man untied the bundle and when he found the man inside it, he asked, "Other people only cut one bundle of firewood a day. Why do you cut two?"
The woodcutter made a bow and replied, "We are penniless. That's why my wife and I cut two bundles of firewood a day. One bundle is for our own use and the other we carry to the market. With it we can buy rice to make porridge."
The old man chuckled and said to the woodcutter in a warmhearted tone of voice, "I've known for a long time that you are a decent couple and lead a frugal and hardworking life. I shall give you a piece of treasure. Take it back with you and it will provide you with your livelihood."
As soon as he had finished speaking, there came seven fairies who led the young man into a magnificent palace. Its golden eaves and gleaming roof tiles shone so brightly that the moment he entered, he could no longer open his eyes. Inside the palace there were many kinds of rare objects on display that he had never seen before. Moneybags of all shapes and sizes hung in one room. The fairies asked him, "Which one do you like best? Choose whichever you please, and take it home."
The woodcutter was beside himself with joy, "I'd like that moneybag, the one full of precious things. Give me that round, bulging one." He chose the biggest one and took it down.
Just at this moment, the white-haired old man came in and, with a stern expression on his face, said to the young man, "You cannot take that one. I'll give you an empty one. Every day you can take one tael of silver out of it, and no more." The woodcutter reluctantly agreed. He took the empty moneybag and, clinging onto the enormous rope, he was lowered to the ground.
Once home, he gave the moneybag to his wife and told her the whole story. She was most excited. In the daytime they went as usual to cut firewood. But from then on, whenever they returned home after dark, they would close the door and open the moneybag. Instantly, a lump of silver would roll jingling out. When they weighed it on the palm of their hand, they found it to be exactly one tael. Every day one tael of silver and no more came rolling out of the bag. The wife saved them up one by one.
Time went slowly by. One day the husband suggested, "Let's buy an ox."
The wife didn't agree. A few days later, the husband suggested again, "How about buying a few acres of land?"
His wife didn't agree with that either. A few more days elapsed, and the wife herself proposed, "Let's build a little thatched cottage."
The husband was itching to spend all the money they had saved and said, "Since we have so much money in hand, why don't we build a big brick house?"
The wife could not dissuade her husband and reluctantly went along with his idea.
The husband spent the money on bricks, tiles and timber and on hiring carpenters and masons. From that time on, neither of them went into the mountain to cut firewood any more. The day came when their pile of silver was almost exhausted, but the new house was still unfinished. It had long been in the back of the husband's mind to ask the moneybag to produce more silver. So without his wife's knowledge, he opened the bag for a second time that day. Instantly, another lump of snow-white silver rolled jingling out of the bag onto the ground. He opened it a third time and received a third lump.
He thought to himself, "If I go on like this, I can get the house finished in no time!" He quite forgot the old man's warning. But when he opened the bag for the fourth time, it was absolutely empty. This time not a scrap of silver came out of it. It was just an old cloth bag. When he turned to look at his unfinished brick house, that was gone as well. There before him was his old thatched hut.
The woodcutter felt very sad. His wife came over and consoled him, "We can't depend on the magic moneybag from heaven. Let's go back to the mountain to cut firewood as we did before. That's a more dependable way of earning a living."
From that day on, the young couple once again went up to the mountain to cut firewood and led their old, hardworking life.

Marhaban ya ramadhan, sungguh suatu kebahagiaan bagi umat muslim karena dapat kembali berjumpa dengan bulan suci ramadhan. Sungguh suasana yang sangat khas, dari segi religius dimana kita berlomba-lomba mendekatkan diri kepadaNya, segi psikolog dimana kita mesti menahan segala hawa nafsu, hingga segi kuliner dimana ngabuburit berburu santapan berbuka menjadi acara yang amat dinanti.

Seakan turut meramaikan hiruk pikuk di bulan Ramadhan, langit berbintang pun akan mempersembahkan pesonanya. Hemm, kiranya apa peristiwa astronomis yang akan terjadi?

Dialah hujan meteor perseids, peristiwa astronomis periodik dimana peristiwa ini akan terjadi jika Bumi melintasi “puing-puing” komet Swift-Tuttle. Saat itu, meteoroid atau “puing-puing” komet akan tertarik gravitasi Bumi, kemudian bertumbukan dengan atmosfer Bumi hingga meteoroid itu akan berpijar menampilkan apa yang disebut meteor.

Hujan meteor perseids dapat diamati sejak tanggal 23 Juli sampai 22 Agustus, namun baru akan mencapai puncaknya pada tanggal 12 s/d 13 Agustus nanti. Pada saat puncaknya diperkirakan akan terlihat sekitar 90 meteor tiap jamnya. Radian atau pusat hujan meteor perseids sendiri berada di rasi perseus, dengan begitu meteor yang berjatuhan “Seakan-akan” berasal dari rasi tersebut.

Untuk melakukan pengamatan, berikut sedikit tipsnya:

Persiapan:

• Tentukan tempat pengamatan. Dimana saja, idealnya dengan medan pandang luas dan langit gelap.

• Tentukan waktu pengamatan. Waktu terbaik adalah setelah tengah malam hingga fajar tiba. Namun mengingat fase bulan menjelang purnama yang kemungkinan akan mengurangi gelapnya langit, ada baiknya pengamatan dilakukan pada tanggal 12 dan 13 Agustus 2011 setelah bulan terbenam, yaitu menjelang subuh (sekitar pukul 4.00 waktu setempat) hingga fajar tiba.

• Siapkan perlengkapan seperti jaket, peta bintang, kamera dan lainnya jika dirasa perlu.

• Tidak diperlukan teleskop atau binokuler.

Pengamatan:

• Pastikan langit cerah.

• Bawa perlengkapan yang sudah disiapkan.

• Mulailah (bersabar) menengadah ke langit di tempat yang sudah direncanakan. Perlu diketahui bahwa meteor jatuh satu per satu sehingga perlu kesabaran untuk mengamatinya.

• Pandangi tiap-tiap sudut langit perlahan. Meski radiannya adalah rasi perseus di langit sebelah timur laut, bukan berarti pengamatan hanya terfokus kesana. Meteor bisa jatuh di langit disebelah mana saja.

Nah, bagaimana? Menarik bukan?
Selamat sahur sembari berburu meteor perseids.