鄭和航海圖之傘型導航星官「華蓋」

鄭和下西洋之航海圖原名《自寶船廠開船從龍江關出水直抵外國諸番圖》，載於明茅元儀之《武備志》卷二百四十，其中的中亞至東非海岸導航針路圖以及天文導航用的過洋牽星圖，皆記錄了星官「華蓋」。然而以天文軟體Stellarium重現鄭和下西洋當時印度洋上的星空，卻發現在「錫蘭山回蘇門答剌過洋牽星圖」中，北辰、織女、燈籠骨星、西北布司星、西南布司星與南門雙星之仰角高度皆可符合牽星圖記錄，唯有傳統星官「華蓋」仍在海平面以下無法觀測。歷來文獻曾對此問題提出多種華蓋可能的位置，其範圍涵蓋了小熊座、仙后座、天龍座、獵犬座到大熊座等。本文整理各家說法，並以牽星圖之星官實際仰角與長期觀星可累積之經驗來指出各家疑點。文中又以天文軟體搭配牽星圖中同高度之記錄，尋得牽星時的「一指」測量誤差可達1.5～2度，一指在1.0至2.5度範圍，可記為1.75±0.75度，並發現牽星不需在特定時間或南十字座位於特定高度之時才能進行。牽星原為輔助導航用，應需越多星可供驗證越好，但以往文獻之各種星座配對原則或特定時刻量測的說法若剛好遇到該星座被雲層遮蔽則失敗風險極高，與牽星基本概念抵觸，應該不是牽星原有的規則。本文最後提出一種新的可能：圖中華蓋所指，應為近似官方華蓋的另一組傘型星，其位置在天龍座範圍內，仰角高度最為符合牽星圖所載。由於傘型之特殊易認，即便亮度不高卻亦可用於牽星。本文名之為「假華蓋」，如同西洋星座中的南十字座附近亦有一個類似分身名為「假十字」一般。

關鍵字

鄭和航海圖；過洋牽星圖；測量誤差；華蓋

並列摘要

The nautical chart of Zheng He's expedition voyages ＂Mao Kun map＂ was originally published in Wubei Zhi Volume 240 by Mao Yuanyi in the Ming Dynasty. At the end of the navigation map from Central Asia to the East African coast and the cross-ocean stellar diagrams used for celestial navigation, all record the Chinese constellation ＂Huagai＂(華蓋) , it means the canopy of the emperor. However, when the sky of the star chart ＂From Ceylon Back to Sumatra＂ was reproduced with the astronomical software Stellarium, all of the stars including Polaris (αUMi), Vega, Crux, Pollux, Procyon, and Rigil Kentaurus can be in line with the records of the star chart, but only the traditional constellation ＂Huagai＂ is still unobservable and below the sea level. Historical documents have pointed out several possible positions of the Huagai, its range covers Ursa Minor, Cassiopeia, Draco, Canes Venatici and Ursa Major. This article sorts out various opinions, and uses the comparison of the elevation angles of the stars in the star charts and the practical experience of long-term stargazing to point out the doubts of each paper. In this article, we used astronomy software with the records of the same altitude in the star charts, find the unit ＂one-finger＂ measurement error will be 1.5~2 degrees, and its range is 1.0~2.5 degrees, which can be recorded as 1.75±0.75 degrees. It is no need to wait for a specific time or when the Crux is at a specific height to measure the elevation angles. Astronomical navigation was originally used for auxiliary purposes, and the more stars available for verification, the better. However, according to the methods of various constellation pairings or measurements at specific moments in the previous literature, if the constellation happens to be obscured by clouds, the risk of failure is extremely high, so they conflict with the concept of celestial navigation, should not be the original rules of Mao Kun map. Finally, this article proposes a new possibility: the Huagai refers to another group of stars that is similar to the official umbrella-shaped constellation, its position is within the range of Draco, and its elevation angle is most in line with the star chart. Due to the special and easy-to-recognize umbrella shape, even non-bright grade star can also be helpful to navigation. Its name in this article is ＂False Huagai ＂, just as there is a similar clone named ＂False Cross＂ near the Crux in the Western constellation.