Mars is too faint and too low in the sky to be observable.
火星太暗，在空中位置太低，所以观测不到。
《柯林斯英汉双解大词典》
2
In contrast, high-frequency fluctuations such as seasonal temperature variations are observable and somewhat predictable, so that groups could have adapted their behaviors accordingly.
Some archaeological sites have always been easily observable—for example, the Parthenon in Athens, Greece; the pyramids of Giza in Egypt; and the megaliths of Stonehenge in southern England.