If you're a relatively broke graduate student studying abroad in an expensive city you can walk through the back gate for free. If you approach the observatory from the back there is a small gate where you can still stand in two hemispheres without giving up a tenner. It's probably where people took pictures before Greenwich installed the big metal sculpture in the first picture, but it makes you feel a little rebellious AND its free. So go this route if you're on a budget.
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