Scientific Space: Important Astronomical Phenomena in April 2010

April 9: Mercury at Greatest Eastern Elongation

For observers on Earth, at 7:00 AM Beijing Time on April 9, Mercury will reach its first greatest eastern elongation of 2010. The best time for us to observe it will be during the twilight of that evening.

The so-called "greatest elongation" refers to the position where the angular distance between Mercury and the Sun is at its maximum, relative to Earth. Observation conditions for Mercury vary greatly across different planets. If astronomical observations were possible on Venus, Mercury’s conditions would be much better because its maximum angular distance from the Sun would be much larger than seen from Earth. On Mars, however, Mercury is always very close to the Sun and is almost impossible to observe. On April 7, the angular distance between Mercury and the Sun reaches its maximum for this eastern appearance at 19^\circ, which seems far from the theoretical maximum of over 28^\circ. However, at this time, Mercury’s declination is nearly 17^\circ, while the Sun’s declination is only about 7^\circ. Therefore, for mid-to-high latitude regions in the Northern Hemisphere, the observation conditions for this greatest eastern elongation are very ideal. At sunset, Mercury appears in the west-northwest direction at an altitude of 17^\circ. By the end of civil twilight (when the Sun is 6^\circ below the horizon), its altitude will still be around 13^\circ. If the weather is clear and your western horizon is unobstructed, finding Mercury (with a brightness of about magnitude 0) should not be a problem.

During this greatest eastern elongation, Mercury is accompanied by a brighter planet: Venus. On the evening of the 9th, the angular distance between Venus and Mercury is only about 3^\circ. Venus, with a brightness of -3.9, serves as an excellent celestial guidepost. Once you find Venus, it becomes much easier to locate Mercury using the provided star charts.

April 22: Mars in Conjunction with the Moon

Throughout the first half of 2010, observation conditions for Mars remain ideal. In April, Mars is located in the constellation Cancer with a brightness of approximately magnitude 0.4. On April 17, Mars will be in conjunction with the Beehive Cluster (M44) for the third time in six months. This time, its angular distance from the center of the cluster is only 1^\circ, which is roughly the same as the apparent diameter of the open cluster itself. This presents a great opportunity to photograph Mars with the Beehive Cluster. Note that the brightest stars in the Beehive Cluster are only around magnitude 6, much dimmer than Mars. You can use a medium focal length lens with a camera on an equatorial mount for long exposures to capture them together.

By April 22, the First Quarter Moon (9th day of the lunar month) will meet Mars, and the conjunction will take place that evening. In fact, a Mars-Moon conjunction occurs every month. Because Mars is currently in a good position for observation, this event receives significant attention. The circumstances of these conjunctions vary; the focus and results of observation or photography differ each time. For this April conjunction, since Mars is three months past opposition, it is nearly at the upper culmination at sunset. Therefore, when the Moon passes near it, it must be a First Quarter Moon. At this time, both are at a high altitude, making it easy to photograph them together, though incorporating interesting foreground elements may be difficult. During the conjunction in late January, Mars was at opposition (a Full Moon), and the event occurred at twilight as they rose in the east, making foreground photography much easier. It is also worth noting that the Moon is much brighter than Mars; in most cases, the Moon will be overexposed when photographing the conjunction. One might think that a conjunction with a crescent moon would avoid this, but by then, Mars’s brightness will have also significantly decreased.

April 23: Lyrid Meteor Shower Peak

After observing the Mars-Moon conjunction, observers can enjoy the peak of the Lyrid meteor shower in the later half of the night. The Lyrids are familiar to astronomy enthusiasts as they are not only the most noteworthy spring meteor shower but also one of the best annual showers for observers in the mid-to-high latitudes of the Northern Hemisphere.

Unlike most other meteor showers that last for over a month, the Lyrids have a very short active period. We can only see meteors belonging to this stream for less than 10 days in mid-to-late April. This year, the Lyrids are active from April 16 to April 25. According to the International Meteor Organization (IMO), the peak is predicted to occur at 1:00 AM Beijing Time on the 23rd, a time slot very suitable for observers in China.

The radiant of the Lyrid meteor shower is located between the constellations Hercules and Lyra. Hercules has few bright stars, while Lyra features the famous magnitude 0 star, Vega, making it easy to identify. Of course, you don’t need to stare directly at the radiant; meteors can appear anywhere in the sky. Taking Beijing as an example, the radiant will rise above the horizon at 8:00 PM on the 22nd, from which point meteors may start to appear. The influence of the First Quarter Moon will last until about 1:00 AM on the 23rd. The two hours following that, until the start of astronomical dawn (when the Sun is 18^\circ below the horizon), are the most likely times for the peak to occur. Since the radiant will be near the zenith before dawn, this year is an excellent opportunity for observation.

It should be noted that compared to showers like the Geminids or Perseids, the Lyrid rate is much lower. Based on data from 1988 to 2000, the ZHR (Zenithal Hourly Rate) during high years was only 23, and in low years it was as low as 14, usually averaging around 18. Historically, only 1982 saw a recorded ZHR of over 90. Although conditions are good this year, a massive surge is unlikely. Furthermore, while the active period is short, the peak is relatively flat and sustained. Therefore, observations should ideally last from radiant rise until dawn, and visual observations are best performed by a team for safety and warmth.

Solar System Information:

Sun: At the beginning of the month, the Sun’s apparent Right Ascension (RA) and Declination (Dec) are 0h 41.0m and +4^\circ 24.9’; at the end of the month, they are 2h 28.6m and +14^\circ 40.4’. This month, the Sun moves from Pisces to Aries. On the 5th at 05:30 is Qingming (Solar Longitude 15^\circ). On the 20th at 12:30 is Guyu (Solar Longitude 30^\circ).

Moon: The Moon reaches apogee on April 9 at 11:00 and perigee on April 25 at 05:00. Lunar phases: Last Quarter on the 6th at 17:37, New Moon on the 14th at 20:29, First Quarter on the 22nd at 02:20, and Full Moon on the 28th at 20:18.

Mercury: Inferior conjunction occurs on the 29th. Before the 29th, Mercury is an evening star appearing in the northwestern sky after sunset. As its distance from the Sun increases, its altitude at sunset and its duration in the night sky increase, improving observation conditions. On April 9, Mercury reaches greatest eastern elongation (19^\circ from the Sun, altitude 17^\circ, magnitude -0.1), the first favorable opportunity of the year. After elongation, it approaches the Sun until the 29th, when it is lost in the Sun’s glare. After inferior conjunction, it becomes a morning star in the northeast. On the 16th at 07:00, Mercury is in conjunction with the Moon (10^\circ south). On the 18th at 18:00, Mercury is stationary.

Venus: Moves from Pisces through Aries into Taurus. As an evening star, it appears low in the northwest after sunset. Its distance from the Sun continues to increase, with its altitude at sunset rising from 17^\circ to 23^\circ. Brightness is about -3.9, with conditions improving. On the 16th at 21:00, Venus is in conjunction with the Moon (4^\circ south).

Mars: Prograde in Cancer. At sunset, it is high in the southeast, moving to the southwest by late April. Setting time moves from 3:00 AM to 1:30 AM. Visibility time decreases daily, and brightness drops from +0.1 to +0.4. Observation conditions remain good. On the 22nd at 17:00, Mars is in conjunction with the Moon (5^\circ north).

Jupiter: Prograde in Aquarius. Rises in the southeast around 4:00 AM. Brightness is -2.1. Its altitude at sunrise increases from 9^\circ to 20^\circ. Conditions are improving. On the 12th at 06:00, Jupiter is in conjunction with the Moon (60^\circ south).

Saturn: Retrograde in Virgo. Located in the southeast at sunset, brightness +0.7. It sets shortly before sunrise. This remains an excellent time for Saturn observation. On the 26th at 08:00, Saturn is in conjunction with the Moon (8.0^\circ north).

Uranus: Located in Aquarius. On the 12th at 22:00, Uranus is in conjunction with the Moon (6^\circ south).

Neptune: Located in Capricornus. On the 10th at 09:00, Neptune is in conjunction with the Moon (4^\circ south).

(Main data sources: "2010 Astronomical Outlook", "Amateur Astronomer", Stellarium, and other internet resources; compiled by Scientific Space)

When reposting, please include the original address: https://kexue.fm/archives/566

For more detailed reposting guidelines, please refer to: "Scientific Space FAQ"