The stars (the celestial variety at least) of Tokyo suffer from the same afflictions of those in almost every major urban centre of the world; the colour of the air and the level of the light we release from the streets to the dark of the night. Air pollution itself is actually not that bad in Japan although at New Year, the views of Fuji-san from the windows of Shinjuku are undoubtedly clearer as the city's population follows the annual exodus to their home towns and the cars and their exhaust disappear from the streets.
The winter months are best for viewing the skies but by April the haze and cloud often obscure what can be an incredible night's light show. The planets and the major constellations are visible but it will have been a long while since the Milky Way was visible from the streets of Ginza. And it's only when you head to the mountains do you realise what you are missing; the nights of the Northern Hemisphere are simply stunning.
At around a thousand meters, (~3,000 ft) the air is so clear the moon will keep you awake. The population of Japan has, since the 1950's, been predominantly urban and so the villages and towns of the rural country are fewer and further between and the skies can be seen in all their majesty. The beer's not bad either... If you have the chance take a trip to an onsen outside of the city, sit and relax with a waters up to your neck and just watch the stars go by. Living in Tokyo you can sometimes forget how incredible they are.
|Moonset, 5.00am, an hour before dawn|