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Look at that Sunspot Go!

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Look at that Sunspot Go!

On November 15, 1999, the planet Mercury will cross the face of the Sun. The unusual grazing occultation will not be repeated for centuries.

Venus, Regulus, and the cresent moon on
Oct 5, 1999November 12, 1999: Solar maximum is just around the corner, which means that the Sun is peppered with sunspots. Just today there were 5 groups and at least 50 individual spots visible on the surface of our star. Sunspots -- cool areas created by twisted magnetic field lines poking through the sun's surface -- move rather slowly. They're usually visible for about two weeks as they move from east to west with the solar rotation.

On Monday, November 15, observers in the Pacific hemisphere can catch a glimpse of a different kind of sunspot -- a black dot that zips across the sun in little more than an hour. It's not really a sunspot; it's Mercury, the nearest planet to the sun!

Right: This white light image of the sun shows several sunspot groups on November 12, 1999. On Monday, November 15, another tiny dark spot will appear briefly near the Sun's northeastern limb when the planet Mercury crosses in front of the Sun.

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The transit or passage of a planet across the disk of the Sun is a relatively unusual occurrence. As seen from Earth, only transits of Mercury and Venus are possible. On average, there are 13 transits of Mercury each century. Transits of Venus are even more rare. They occur in pairs with more than a century separating each pair.

On 1999 November 15, Mercury will cross the visible disk of the Sun for the first time since 1993. At approximately 2115 UT (4:15 p.m. EST) the black disk of the planet will appear at the Sun's northern limb, about a third of the way around from North to East. These cardinal directions are easy to figure by simply nudging an equatorial mounted telescope back and forth on both axes. The black disk of the planet will be small -- 9.9 arcseconds across -- and blaker than any normal sunspot.

Monday's transit is an especially unique event called a grazing transit. Mercury's motion across the Sun will follow a short path near the Sun's northeastern limb. From some parts of the Earth observers will see only part of Mercury's black disk superimposed on the Sun. From other observing sites Mercury's entire disk will be visible just inside the sun's northern limb. The next grazing transit like this one won't happen until the year 2314!

The partial phase of the transit will be visible from most of Australia, New Zealand, and Antarctica. The total transit will be visible from Papua-New Guinea, northeastern Australia, Hawaii, western South America and most of North America, where the event occurs shortly before sunset. Although we say that the transit will be "visible," you should never look directly at the sun. Instead, use eyepiece projection or a telescope with suitable solar filters.

Geocentric Contact Times 1999 Transit of Mercury
Event             UT        
Contact I      21:15:01
Contact II     21:29:44
Greatest       21:40:53
Contact III    21:52:03
Contact IV     22:06:4

Due to the nature of the 
grazing transit, the actual 
contact times may differ by 
tens of minutes depending on 
the observer's actual 
geographic coordinates.

Timing Predictions for
the U.S. and Canada

Timing Predictions for
Rest of the World

All transits of Mercury fall within several days of 8 May and 10 November. Since Mercury's orbit is inclined seven degrees to Earth's, the Earth crosses through Mercury's orbital plane twice each year - on those dates. If Mercury passes between the Earth and the Sun (astronomers call this "inferior conjunction") at that time, a transit will occur. During November transits, like this one, Mercury is near its furthest distance from the Sun (aphelion) and exhibits a disk only 10 arc-seconds in diameter. By comparison, the planet is near its closest approach to the Sun (perihelion) during May transits and appears 12 arc-seconds across.

The first hint of Mercury's ingress onto the solar disk is called first contact (or Contact I). Second contact (Contact II) occurs at the instant that the entire disk of Mercury is visible against the Sun. Third contact (Contact III) takes place just as Mercury's disk touches the solar limb on its way to exit the solar disk. Finally, fourth contact (Contact IV) is when the last trace of the planet disappears. Accurate timings of these contacts can have scientific value.

Edmund Halley first realized that transits could be used to measure the Sun's distance, thereby establishing the absolute scale of the solar system from Kepler's third law. Unfortunately, his method is somewhat impractical since contact timings of the required accuracy are difficult to make. Nevertheless, the 1761 and 1769 expeditions to observe the transits of Venus gave astronomers their first good value for the Sun's distance.

Nowadays the distances to the Sun and to Mercury are known very precisely. Planetary transit timings have very little to offer for improving these values. However, there is another mystery that such timing measurements can address. There is growing evidence that the sun's radius varies by a fraction of an arcsecond during the solar cycle. Accurate timings, especially from a grazing transit like the one on November 15 can be used to calculate the radius of the Sun.

Since Mercury is only 1/194 the Sun's apparent diameter, a telescope with a magnification of 50x to 100x is recommended to watch this event. Naturally, the telescope must be equipped with proper filters to ensure safe solar viewing. NEVER look directly at the Sun! For information on solar filters and safe observing methods click here.

The visual and photographic requirements for observing a transit are identical to those for sunspots. The most valuable scientific contribution the amateur can make is to time the four contacts at ingress and egress. Observing techniques and equipment are similar to those used for lunar occultations. Observing tips may be found at the Association of Lunar and Planetary Observers website (click here). A telescope equipped with a solar filter and a CCD video camera would be ideal for timing observations.

To make your observations useful to scientists, you'll need to record an accurate time signal on the audio track of the video recorder. The easiest way to do this is to tune a shortwave receiver to WWV, which transmits time signals at 2.5, 5, 10, 15, and 20 MHz, and position the receiver within listening distance of your camcorder's microphone. To hear what a WWV signal sounds like, you can phone (303) 499-7111. Another good source of time information is the Canadian radio station CHU, which broadcasts at 3.330 MHz and 7.335 MHz.

Transits of Mercury: 1970-2050

Date          UT    Separation*

1970 May 09   08:16    114"
1973 Nov 10   10:32     26"
1986 Nov 13   04:07    471"
1993 Nov 06   03:57    927"
1999 Nov 15   21:41    963"
2003 May 07   07:52    708"
2006 Nov 08   21:41    423"
2016 May 09   14:57    319"
2019 Nov 11   15:20     76"
2032 Nov 13   08:54    572"
2039 Nov 07   08:46    822"
2049 May 07   14:24    512" 

* separation is the distance 
(arc-seconds) between the 
centers of the Sun and Mercury
When reporting your observations, it is also important to specify your latitude and longitude as accurately as possible. Ideally the coordinates should pin down your location to within 15 ft. The best way to locate your observing site is with the aid of a GPS receiver. Unfortunately, these are somewhat expensive and not widely available. Another approach is to carefully measure topographic survey maps. In the USA, suitable maps can be ordered directly from the US Geological Survey by calling 1-800-USAMAPS. There are also mapping resources online. For instance, if you are observing from an urban area you can search for your address on In addition to a map, it will return a decimal latitude and longitude.

If you feel that you have made accurate and/or interesting observations, you may contact Dr. Tony Phillips, who can help by forwarding them to the appropriate scientists. Pictures or video of the transit are also welcomed for use in a future Science@NASA headline.

The author acknowledges Fred Espenak's excellent web site on the Mercury transit for much of the information that appears in this article.

Web Links

1999 Transit of Mercury -- by Fred Espenak of the NASA/Goddard Space Flight Center

This week's sky at a glance -- from Sky & Telescope

Jack Star Gazer -- Jack Horkheimer's naked eye astronomy web site

Mercury - from the SEDS Nine Planets web site

The Sun - from the SEDS Nine Planets web site

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For more information, please contact:
Dr. John M. Hor , Director of Science Communications
Author: Dr. Tony Phillips
Curator: Linda Porter
NASA Official: Frank M. Rose