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Carter Observatory: The July Night Sky

The Night Sky

Brian Carter



The July Night Sky

We have passed the longest night for this year and therefore the days will start to lengthen, but this will not be noticeable until towards the end of July.

Planets
July is an excellent month for viewing the planets. Venus, Mars, Jupiter and Saturn will be visible for all of the month. Mercury will be visible for all but the very start of the month.

Venus will be visible in the Western evening sky. At the start of the month it sets at 20 34 and at 19 40 by month’s end. This is the last full month this year that Venus will be in our evening sky. Venus starts the month in the constellation of Leo, moving into Sextans on July 30. Its brilliant magnitude starts and ends the month at –4.4, peaking at –4.5 in the middle of the month.

Saturn will be visible in the early evening in July. At the start of the month it sets at 20 33 and at 18 53 by month’s end. Saturn is in the constellation of Leo, in which it remains until September 2009. Its magnitude is a constant 0.6 during the month.

Jupiter will be visible for all but the very end of the night. At the start of the month it sets at 05 54 and at 03 47 by month’s end. Jupiter is in the constellation of Ophiuchus, in which it remains until 2007 December. Its magnitude slightly fades from –2.5 to –2.4 by the end of July.

Mars will be visible for the last quarter of the night. At the start of July it rises at 02 35 and at 02 24 by month’s end. Mars start the month in the constellation of Aries, moving into Taurus on July 29. Its magnitude slightly brightens from 0.7 to 0.5 during the month.

Mercury will be visible in the Eastern morning sky for all but the start of July. At the start of the month it rises at 07 16, at 06 14 by July 19 and at 06 38 by month’s end. Mercury starts the month in the constellation of Gemini, moving into Orion on July 3 and finally back into Gemini on July 19. During July it very rapidly brightens from 4.7 to –0.9.

All times are for Wellington unless otherwise stated. Other centres may vary by a few minutes.

Phases of the Moon

Full Moon – July 1 at 01 49.
Last Quarter – July 8 at 04 54.
New Moon – July 15 at 00 04.
First Quarter – July 22 at 18 29.
Full Moon – July 30 at 12 48.

Note the two Full Moons in July. This is known by some people as a blue Moon month. More about this subject later.

Earth at Aphelion on July 7

The Earth is at aphelion (furthest from the Sun) at 12 00 on July 7. The distance is 1.0167059 AU, which is 152,097,040 km. The corollary of the above is to say that the Sun is at apogee (Sun furthest from the Earth).

Blue Moon in July

July is a Blue Moon month as it has two Full Moons, the first on July 1 at 01 49 and the second on July 30 at 12 48. Blue Moons happen every 2½ to 3 years, hence the phrase “Once in a Blue Moon”.

This phenomenon occurs in New Zealand in July but because of the various time zones, it happened in May and June in other countries.

As far as we can tell, the original Blue Moon definition comes from the Maine Farmers’ Almanac, which started publication towards the beginning of last century. This almanac’s calendar ran from one Winter (Northern hemisphere) Solstice to the next. Each year was divided into four fixed seasons of equal length. Most years contain 12 full Moons - three each in Winter, Spring, Summer, and Autumn - and each was named for an activity appropriate to the time of year (such as the Harvest Moon and specific Moons associated with Easter and Christmas). But occasionally the year contained 13 full Moons so one of the seasons had an extra full Moon. The third full Moon in the season was called a Blue Moon. The third one got this honour, because only then did the names of the other full Moons fall at the proper times relative to the Solstices and Equinoxes.

Now comes the mistake. An article, by James Hugh Pruett, entitled “Once in a Blue Moon” appeared in the March 1946 edition of Sky & Telescope. Pruett was an amateur astronomer and frequently wrote articles on a variety of topics, especially fireballs and meteors for Sky & Telescope. The article mentions, as a basis, the Maine Farmers’ Almanac, but unfortunately, in the article he says:

“Seven times in 19 years there were - and still are – 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon”.

Pruett must not have had the almanac handy, or he would have noticed that the almanac’s Blue Moon fell on the 21st of the month (obviously not the second full Moon that month). This new erroneous definition of Blue Moon crept into folklore, presumably because Sky & Telescope was more widely read and authoritatively accepted than the Maine Farmers’ Almanac.

Another accepted meaning of the term Blue Moon is when certain atmospheric conditions cause the Moon to take on a bluish colour. Dust in the atmosphere usually turns the Moon a reddish colour.

Webster’s Dictionary does not mention the two Full Moon explanation. The entry says, “the rare blue appearance of the moon that is due to dust particles in the high atmosphere”. Blue Moon is not even mentioned in the Oxford English Dictionary or the Encyclopædia Britannica.

Wikipedia agrees with the explanations above. http://en.wikipedia.org/wiki/Blue_Moon

May Sky Chart

This chart shows the sky as it appears at about 21:00 for ~July 15.


Click for big version


How To Use the Sky Charts
To use the sky chart hold it up to the sky so that the direction in which you are looking is at the lower edge of the map. For example, if you are looking at the western horizon then the map should be held so that the “WEST” label is at the lower edge. The altitude and direction of the stars and planets will then be correctly shown. The centre of the chart will be directly overhead.


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Brian Carter is the Senior Astronomer at Carter Observatory (The National Observatory of New Zealand), PO Box 2909, Wellington. (DDI; 04 494 8321, Email: Brian.Carter@carterobservatory.org , Observatory Web Site: www.CarterObservatory.org )

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