National Astronomical Observatory of Japan

April, 2015


The sky of Tokyo

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

Calendar (April)

4Total lunar eclipse (able to observe throughout Japan) / Full Moon
6Uranus at conjunction
9Jupiter at stationary point
10Mercury at superior conjunction
12Last Quarter
19New Moon
22Peak of April Lyrids Meteor Shower (There is almost no effect of moonlight)
26First Quarter
29Day of Showa [Showa no hi] (National holiday)

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


Because Mercury is at superior conjunction on the 10th, in the beginning and middle of the month it’s positioned close to the Sun and it can’t be observed. Towards the end of the month it’s elevation in the western evening sky slowly increases, reaching its greatest eastern elongation on May 7th. It is very easy to observe from the end of April to the first part of May.
Venus shines brightly in the eastern sky after the Sun sets. The brightness is Magnitude -4.0 to -4.1.
Because Mars’s position is close to the Sun, and its brightness is dim, only Magnitude 1.4, it is difficult to observe.
Jupiter is located in the constellation Cancer. After sunset, look high in the south-western sky. This is a good chance to observe it. The brightness is Magnitude -2.3 to -2.1.
Saturn is located in the constellation Scorpio. For Tokyo, at the start of the month it rises at about 23:00, shifting to around 21:00 by the end of the month. The brightness is Magnitude 0.3 to 0.1.

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


Total Lunar Eclipse

The appearance of the April 4th Total Lunar Eclipse as seen from Tokyo
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In April, the conditions are good for the total lunar eclipse; all of Japan can see the entire eclipse from start to finish. A total lunar eclipse is when the Moon passes through Earth’s shadow, darkening from the edge as if disappearing; before long the entire Moon enters the shadow. For this lunar eclipse, the Moon starts to disappear at 19:15; the Moon is totally eclipsed for 12 minutes from 20:54 until 21:06; and the eclipse ends at 22:45. The end is a little late, but otherwise this eclipse occurs at a time that is relatively easy to observe. Please take a look.

You can enjoy the lunar eclipse with just your unaided eyes. If you have a telescope or binoculars, you can also try observing the eclipse with one of those. Also, during the total lunar eclipse, you can see faint stars without interference from the moonlight. This is another way to enjoy the total lunar eclipse.

The Moon Passing Close to Venus

About 19:30, April 22, 2015
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Venus, the Evening Star, shines brightly in the western sky after sunset. Since it is very bright, approximately Magnitude -4, it draws the eye.

The crescent moon appears to pass close to Venus on the 21st (Moon phase 2.6 days) and the 22nd (Moon phase 3.6 days), giving us a beautiful spectacle. Winter’s 1st Magnitude stars, first and foremost Aldebaran to the lower left of Venus in the constellation Taurus, color the surroundings.

Source: Ephemeris Computation Office, NAOJ

With the NAOJ Ephemeris Computation Office’s “Sky Viewer,” you can easily investigate how to spot the Moon and planets in a typical urban night sky.

An Opportunity to Observe Jupiter

An Opportunity to Observe Jupiter
Click to enlarge

This is a good chance to observe Jupiter which reached opposition on February 7th.

About the time the sky gets dark after sunset, Jupiter can be seen in a high, extremely visible location a little west of due south. It is very bright, about Magnitude -2. Since it is the second brightest object after Venus, which is low in the western sky, it is easy to spot.

On the 26th, Jupiter appears close above the first-quarter moon. After that, on the 27th and 28th, the Moon moves close to the 1st Magnitude star Regulus in the constellation Leo to the east of Jupiter. Please enjoy watching the position of the Moon with respect to the bright stars change from day to day.

Source:Ephemeris Computation Office , NAOJ

With the NAOJ Ephemeris Computation Office’s “Sky Viewer,” you can easily investigate how to spot the Moon and planets in a typical urban night sky.

Mutual Phenomena of the Jovian Moons

A chart showing the dates when the various phenomena occur and the relative positions of Jupiter and its Moons.
Click to enlarge

Jupiter’s 4 brightest moons are called the “Galilean Moons.” In order starting from the closest to Jupiter, their names are Io, Europa, Ganymede and Callisto. From August 2014 until August 2015, there are events where one moon hides another from view, and times when one moon enters the shadow of another moon. These conditions occur only once every 6 years.

Among the phenomena occurring in April, here we introduce 3 which are easy to observe. They are all events where the shadow of Io, the Galilean Moon closest to Jupiter, covers Europa, the second closest. It is exactly like a lunar eclipse, but because Io’s shadow is smaller than Europa, it becomes an “annular eclipse.” People with telescopes should try to observe it. (This can’t be seen with the naked eye. It could probably be observed with binoculars fixed on a tripod.)

Because the moons are very small, even using a telescope, you can’t see the shape change or the ‘annular eclipse.’ Instead you can watch the brightness change. Europa dims as it enters Io’s shadow. It is its dimmest when it reaches the ‘annular eclipse’ phase. After that, Europa returns to its original brightness. Compared to the brightness before the phenomenon starts, the darkest time is 2 Magnitudes dimmer. You can discern the difference even without using a special instrument. In order to clearly capture the change in brightness, you should start observations several minutes before the phenomenon.

Duration5.1 min5.2 min5.3 min
‘Annular Eclipse’ Duration0.6 min0.7 min0.5 min
Decrease in Magnitude1.882.551.90