National Astronomical Observatory of Japan

December, 2015


The sky of Tokyo

The sky of Tokyo (8:00 p.m. Mid-December)
Click to enlarge

Calendar (December)

3Last Quarter
11New Moon
15Peak of Geminids Meteor Shower around 3 a.m. (The condition is best because the time of peak is in the night, and there is no effect of moonlight)
19First Quarter
22Winter solstice [Toji] [22nd solar term] (Sun’s ecliptic longitude 270°)
23Emperor’s Birthday [Tennou Tanjyobi] (National holiday)
25Full Moon
26Uranus at stationary point
29Greatest Eastern Elongation of Mercury

The posted peak day and time of meteor showers is based on the prediction of IMO(International Meteor Organization).


Mercury reaches its greatest eastern elongation on the 29th, but because its elevation in the western sky is less than 10 degrees at 30 minutes after sunset, it is difficult to observe. In the last part of the month, its brightness is -0.6 magnitude to -0.4 magnitude.
Venus stands out spectacularly, shining brightly in the eastern sky before sunrise. Towards the end of the month, its elevation gradually decreases. The brightness is -4.2 magnitude to -4.1 magnitude.
Mars, located in the constellation Virgo, appears between Venus and Jupiter. It can be seen fairly high in the southwestern sky before sunrise. Its brightness is 1.5 magnitude to 1.3 magnitude.
Jupiter is located in the constellation Leo. It rises above the eastern horizon around midnight. Before dawn, it can be seen in the southern sky. Jupiter’s brightness is -2.0 magnitude to -2.2 magnitude.
Saturn can’t be seen during the beginning and middle of the month because its apparent position is close to the Sun. In the last part of the month, its form reappears in the southeastern sky before dawn. In the last part of the month, its brightness is 0.5 magnitude.

Source: Ephemeris Computation Office, NAOJ

With the “Sky Viewer” you can easily explore the appearance of a typical urban night sky (planets and constellations are visible).The Celestial Phenomenasection of the glossary explains the planetary phenomena terms: greatest elongation, opposition, conjunction, stationary, etc.


Planetary Phenomena


The Moon Passing Close to Jupiter, Mars, and Venus

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Before dawn, 3 planets are lined up in the southeastern sky. The bright one near the horizon is Venus; above and to the right of that Mars draws the eye with its red color and moderate brightness, about 1.5 magnitude; further removed, the planet not quite as bright as Venus is Jupiter.

From the 4th to the 8th, the Moon appears close to these 3 planets. The Moon is the closest to Jupiter on the 4th, the closest to Mars on the 6th, and the closest to Venus on the 8th. Seen from North America, the Moon covers Venus, causing an “Occultation of Venus.” Unfortunately this can’t be seen from Japan.

Please get up early and watch the Moon get thinner day by day as it passes through these planets.

Maximum of the Geminids Meteor Shower

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The Geminids are a meteor shower which can be reliably seen each year. One of its characteristics is that meteors can be seen all night long.

This year, the Geminids are expected to reach their maximum* around 3:00 on the 15th (the night of the 14th). Because the 11th is the new moon, there is not any obtrusive moonlight. For these reasons, Japan has extremely good observing conditions. If you observe from a location with dark skies, you can probably see more than 50 meteorites in an hour.

This is a very cold season, so please take proper measures against the cold when you go observing.

*The part of the meteor shower with the most vigorous meteor activity and the time when this occurs are referred to as the maximum.

Jupiter and the Moon passing close together on New Year’s Eve

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If you look low in the eastern sky around midnight on New Year’s Eve, you can see Jupiter shining brightly right next to the Moon.

After that, the Moon and Jupiter rise in elevation as time passes. Around the time the sky gets light, they are close to due south. During this time, the relative positions of Jupiter and the Moon gradually change.

From time to time, look up at the sky and enjoy the sight of Jupiter accompanying the Moon.