0309_(Translation Practice) 沉船中發現世上最早航海星盤

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Mar 9, 2018 18:16 EN-to-TC translation
This is a practice English-Chinese translation.
Please let me know your opinions and suggestions about this translation.
I would value them highly.
Thanks a lot in advance.
Source (text in the original language):

The slim bronze disc was discovered during the excavation of the Esmeralda shipwreck – a Portuguese vessel that was exploring the Indian Ocean when it sank in a storm in 1503.
這微小青銅圓盤是在挖掘Esmeralda 的船隻殘骸時發現的,Esmeralda 是葡萄牙船隻,1503年失事沈船,當時正在印度洋探險旅途中。

The object is decorated with a royal coat of arms, but laser scanning also revealed faint etchings around its edges, each separated by five degrees. It confirms that this is an astrolabe - an instrument used by mariners to measure the height of the sun above the horizon, enabling them to navigate the seas.

Only around one hundred of these items have been found, and this one most likely dates to the late fifteenth century, making it the earliest known example by several decades.

NOTE:I am not quite sure what "by several decades." means. Would somebody explain it to me? Thanks in advance!