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建造一個合理比例的太陽系模型是大難題，要讓行星有較可觀的體積，行星的軌道就大得難以想像；相反，把模型放置一個局限的空間內，所有行星可能都小到難以察覺到。假如大家有太陽系的數據，大可嘗試對以下的太陽系模型做一些運算。首先把原為1,390,000千米直徑的太陽縮小至只有7米大，並把這個仍然相當可觀的火球放置於太空館的毗鄰，你會發現最接近太陽的4顆行星全位於油尖區內，即使巨大的木星也仍可置身於旺角區，其他的行星都變成新界居民。用這個比例計算，行星有多大？原來水星只有芝麻般大小，金星和地球就如鉛筆頭上的橡膠粒，火星就像一粒綠豆。至於太陽系的兩顆巨人行星就只是兩個普通的蕃茄而已，較吸引的莫如扮演天王星和海王星的兩顆葡萄！附圖是把太陽系各星體的大小以另一尺度去比較。
How big is the Solar System?
To make a scale model of our Solar System is a difficult project. It is very difficult to decide on a reasonable scale for all the orbits and the planets. Their measurements are too extreme. For example, try reducing the sun, which is about 1,390,000 km in diameter, to a ball of fire merely 7 meters across and put it next to the Space Museum. The whole solar system, including poor rejected Pluto, could just fit inside the territory of Hong Kong. The four planets nearest to the sun could be found within Tsimshatsui and Yaumati. Even giant Jupiter is well within Mongkok. All the other planets found themselves in the New Territories. How big would the planets be at this scale? Tiny Mercury would be no bigger than a sesame seed and the king-sized Jupiter would just be a common tomato. ■甄枝強 資深天文愛好者
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