Born and raised in a suburb of a major urban city, I was familiar with a few of the brighter stars in the night sky, but it wasn’t until I saw it for the first time away from all the light pollution that I was taken away by the raw beauty and majesty of it all.

With this short anecdote, what I’m trying to convey is that it’s almost impossible to imagine that any of the various world’s pre-modern cultures never looked up, or were never struck by the beauty of the sky, or otherwise just never came up with various practices and behaviours based on what they saw.

On a brief tangent, although entirely unrelated to history, I really want to plug lightpollutionmap and darksitefinder, which are amazing resources for finding a nearby dark site to see the stars for yourself. It’s worth seeing at least once in your lifetime.

Returning to history, in the sense of curiosity and exploration as I alluded to above, it’s fair to say that astronomical knowledge ran far and deep along the Japanese archipelago. This answer surrounding ‘knowledge’ comes with several caveats.

The use of inverted commas here is, I feel, necessary in giving a full-bodied answer to your question. It’s important to distinguish between ‘knowledge’ in the context I gave previously, which is one comprising a diverse body of beliefs influencing and influenced by both religion and myth (and thereby the very cultural fabric of that society), and ‘knowledge’ in the scientific context, whereby social and scientific understanding is furthered by the pioneering works of such individuals as Galileo, Newton, and Kepler to name but a very few.

In the latter framing, it is fairly well established and agreed upon that scientific astronomy in Japan was entirely ‘imported’, first from China and Korea, and then from Europe. Although there was at one point much discussion over the possibility of an ancient native calendar due to the different dates found in the kojiki and nihon shoki, it seems this point has been largely debunked as nothing more than “careless omissions made in the copying of Chinese calendrical indexes, [and when corrected for,] the system appears to be purely Chinese”. Thus, from the latter point of view, the answer to your question is that “essentially zero” astronomical knowledge (of the scientific breed) was present in Japan prior to European contact.

Conversely, if we use the more generous definition of ‘knowledge’ including any information, skills, or understanding acquired through experience or education, then we can really start to explore the social and cultural aspects of astronomy as it was historically treated in the Japanese archipelago. For the purposes of this ethnocultural approach outlined above, I’m adopting Akira Goto’s habit of referring usually to the ‘Japanese archipelago’ and not ‘Japan’, as the latter is a rather geopolitical notion that obscures the diverse cultural beliefs and traditions across the many islands, such as the mythology of the Okinawan people or the Ainu inhabiting Hokkaido.

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Although the previous definition I gave is more open-minded in many ways, it also leads us down a path where it’s easy to stray dangerously into the territory of archaeoastronomy and ethnoastronomy, both of which I know little about. For this part, I’ll point you to several texts which I understand to be good starting points for reading, and which I also understand to be reputable in general. I’m also heavily relying on them for this answer as well.

Steven Renshaw & Saori Ihara’s “A Cultural History of Astronomy in Japan” discusses several examples of the interplay between the native Shinto beliefs and the night sky, including how the stars of Orion’s belt, Hyades, and Pleiades informed festivals, myths, and the farming and fishing practices of the early inhabitants of the archipelago.

Akira Goto’s “Cultural Astronomy of the Japanese Archipelago” is a more recent text which (among many of its qualities) has a good treatment of astronomy as it was seen by the Ainu in Hokkaido and the Okinawan people of Ryukyu, as well as pre-historic astronomy in the Jomon and Yayoi periods of Japan.

Nakayama Shigeru’s “A History of Japanese Astronomy” looks entirely at the Chinese influence on Japanese astronomy, and is what a large part of the remainder of this answer is based on. It is broken down into two parts – the first is Chinese hegemony in the 7th to 9th centuries, and the second is the decline of these Chinese cultural institutions from the 10th century onwards. As it focuses mainly on written records, it’s understandable why there is not much treatment on pre-Chinese Japan, as the vast bulk of writing in Japan was imported from China and Korea.

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According to Nakayama, the Japanese imported wholesale much of the developments of China’s astronomical schools. Timekeeping, water clocks, and observatories were all introduced in the 7th century, followed soon after with regulations for education and administration of astronomy, astrology, and calendar creation. With these technological and political constructs came also the Chinese cultural beliefs and Buddhist religious practices regarding the heavenly bodies, which had to (in many different places) be reconciled with the native Shinto beliefs already present in mainland Japan. For these examples though I would refer you to the first or second of the above texts I mentioned.

With the collapse of the Tang dynasty in China and the resulting unrest, imperial missions from Japan ceased, and the development of astronomy languished until the late 16th century with the Jesuit influence flowing from Europe, and further on with the peace ushered in by the Edo period.

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Here’s a short answer to the second part of your question, on the naming of the planets.

We know that Chinese astronomers discovered the five visible planets from Earth – Mercury, Venus, Mars, Jupiter, and Saturn. They were as named in Chinese, 水星, 金星, 火星, 木星, and 土星, and when imported as kanji to Japan, would remain named as such (that the early Chinese astronomers named them in this way is beyond the scope of this question). In a recent paper, “Astronomy and Calendrical Science in Early Mikkyo in Japan: Challenges and Adaptations” auth. Jeffrey Kotyk, also finds mention of Venus in the Digest of Classifications of Japanese Words. Here, Venus is given the indigenous reading yūzutsu 由不豆々. Nevertheless, Kotyk emphasises that this word does not “denote or necessitate full knowledge of the plants and their orbits”, and (entirely my speculation here) it could be possible that this name was simply given to the phenomenon of Venus, being the brightest object in the sky barring the sun and moon, appearing and being observed in the sky with regularity.

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In sum, the primary introduction to astronomy was via contact with China and Korea mid-way through the first millennium, and included the technological innovations, cultural beliefs, and religious and spiritual practices of the latter cultures.

Prior to this, written record is scarce and further investigation treads the fence between history and the realms of archaeology and anthropology, although we can still see several social constructs, such as the native Shinto beliefs and mythology, through which the above ideas were filtered into the Japanese archipelago.

Following the end of relations with Tang China, Japanese involvement in astronomy, scientific or otherwise, was minimal until well after European contact in the 1500s.

Nevertheless, there appears to be a rich ethnoastronomic heritage that pervades much of modern Japanese astronomy, despite its focus (and indeed international focus) on Western developments in the science. From the history of written records, little exists of the former, and (paraphrasing Renshaw & Ihara who paraphrase Aveni’s 1989 Introduction “Whither archaeoastronomy?”) we would be better rewarded from looking at the archaeology and anthropology of Japan’s cultural astronomy, as only then do we see what was important to the people we study, and not what is important to us (and in particular, the Western perspective).