夜 空(night sky) を　見 上げ(look up) 　一人(alone)
yo zora wo mi age hitori
Looking up at the night sky alone,

ほうき 星(shooting star)　を　見た の(saw)
houki boshi wo mita no
I saw a shooting star.

The first line starts with the describing the night sky. As it is usually cooling at night, a slight chill can be felt.

Looking up at the sky can be romantic, but when being alone is mentioned, there is some sadness. It could be that we feel sad for the person or that the person is sad, because to mention being alone could mean that there was or were, someone or some people to look at the night sky with.

The night sky feels lonely when compared to the sky in the day, not only because it is dark, but also because the sun isn't there. When it is colder, it is easier to feel lonely, since there is no warm person you can reach out to, there is no heated argument, and no warm comfort in your heart that there is someone beside you.

What some people do when they see a shooting star, is to make a wish before it disappears. When a shooting star appears, there will be people sharing about it to their friends nearby so that they can make a wish together.

A shooting star lasts only for a moment. It is also a moment to treasure.

This first sentence seems to be saying, "In this moment when I look up at the night sky and saw a shooting star..."