Posts Tagged ‘solar’

Unlike the Venus transit which is a rare event, there is a Mercury transit every 7.5 years (on average). Between 1601 and 2400, there are only 14 Venus transit. If you have not seen the last one on June 06, 2012, it is too late  since the next Venus transit occurs on Dec 11, 2117 !! Therefore, we should focus on the Mercury transit.

The next transit of mercury occurs on May 09, 2016. It starts at about 11:12 UT and ends on 18:42 UT. The transit is visible in most of Asia, Europe, Africa, and America. Note that the apparent size of the solar disk is about half a degree while the apparent size of mercury disk is only 8 arc seconds (1 degree = 60 arc minute = 3600 arc second).

The next mercury transits in this century are the followings (time in UT):

Date                    start           max          end

Nov  13, 2032      06:41         08:54      11:07

Nov 07, 2039       07:17         08:46       10:15

May 07, 2049       11:03         14:24       17:44

Nov 09, 2052       23:53         02:29       05:06

May 10, 2062       18:16         21:36       00:57

Nov 11, 2065        17:24         20:06      22:48

Nov 14, 2078        11:42         13:41      15:39

Nov 07, 2085        11:42         13:34      15:26

May 08, 2095        17:20         21:05      00:50

Nov 10, 2098        04:35         07:16      09:57

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Solar battery chargers

Recently, there are quite a lot of solar battery chargers. Such instruments like the one in the following image are able, in theory, to recharge mobile devices, ipods, etc when they are charged, and charge themselves using the sun light.

In this particular example, the company provides the following specifications for the instrument: it has a charge capacity of 3.5 Ah (Ampere  hour) at a potential difference of 3.7 Volts. That means it contains 12.95 Watt h energy when it is fully charged. Just for comparison, a small auto battery has like 40 A.h at a potential difference of 12 V.  That means the energy stored in the device is like 2% of the energy stored in an auto battery, so it is quite a bit actually.

Needless to say, you can charge it via AC adapter or USB cable of your laptop and use it to charge your mobile phone when required. The life cycle according to the webpage is 500. Indeed, the typical Lithium-ion polymer batteries have a life cycle of 1000 or more. Perhaps due to irregular solar charging rather than the standard net charging, the life cycle will be shorter as expressed in the webpage.

Can it really charge itself via solar radiation? I try to simply evaluate how much solar energy it can absorb, according to the data given in the webpage. The solar constant, the energy that the unit area on the top of the earth atmosphere receives in a second is about S = 1400 Watt per square meter. On tropical latitudes, the received energy on the ground is larger than 1000 W/m2. On middle latitudes like central Europe, and in a summer day, S = 800 W/m2. In winter, it is usually about 500 to 600 W/m2 in a sunny day. When it is overcast and very dark, it drops to values comparable to 1W/m2.

Now let us calculate how long it takes to collect 12.95 Wh energy from the received solar radiation on an average day. The collecting area, according to webpage, is 11.5 x 6.0 cm2 , and the efficiency is 17%. If I take S = 300 W/m2 for a relatively bright day, then the rate of collecting energy is

Area  x  Solar constant  x  efficiency = 69 cm2 x 300 W/m2 x 0.17 = 0.35 Watt.

Therefore to collect 12.95 Wh, i.e. to recharge the battery via sun light,  one has to keep the instrument in sunshine for 37 hours. In tropical regions, this time can be a factor 2 or 3 shorter.

This simple calculation shows that if you have such an instrument, you should be patient to get it charged using its own solar panel. In practice, the charging time can be longer because of the cloudy sky, and a decline of the efficiency with aging. 

Do you save money if you buy it?

Perhaps not, specially if you live in high latitudes. The instrument can, in theory, work for 500 cycles of 12.95 Wh. That amounts to 6.5 kWh during its lifetime. The electricity price in expensive cases is like half a dollar per kWh. That means you collect in total like 3.2 US dollar while the instrument costs like 40+ dollars. This particular device is more a (big) spare battery than a solar charger.

There are better options. Consider a similar instrument with an area of 2-3 times larger. That efficiently reduce the recharging time. In addition, I guess the systems with rechargeable AA batteries have the ability to live longer: you can exchange those batteries when they are dead. The solar panels usually work much longer (like 10+ years).

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Green Flash

Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot or rim is visible above the sun.  The reason is that for different part of the solar spectrum, i.e., different colors, there are different refraction index in the atmosphere. That means the earth atmosphere plays role of a prism. This phenomena is called the differential refraction.

The difference between the refraction index of green and red is very small. However, when the sun is very close to the horizon, the red and green beams are off by a few arc seconds. Certain atmospheric conditions can intensify this effect.

Green rim above the solar image seconds before the sunset.

In the above image, you see a green rim above the sun, seconds before the sunset.  It was captured with similar setup as I explained in a previous pots:  a Canon EOS 400D camera was attached to the Celestron Nexstar 4 inch Maksutov telescope in prime focus mode.

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The above photo was captured during the evening of June 15, 2011 when I was waiting for the lunar eclipse. A Canon EOS 400D camera was attached to the Celestron Nexstar 4 inch Maksutov telescope in prime focus mode.

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Flare diagram from GOES x-ray data. The largeest peak is the second X-class flare in the current solar cycle (24).

Flare diagram from the GOES x-ray data. The largest peak is the second X-class flare in the current solar cycle (24).

Explosive events on the solar surface release energy in various forms, e.g., accelerated particles, bulk mass motion, and radiated emission in various spectral ranges.  A solar flare is an explosion on the Sun that happens when energy stored in twisted magnetic fields (usually above sunspots) is suddenly released.  Flares produce a burst of radiation across the electromagnetic spectrum, from radio waves to x-rays and gamma-rays. In a typical flare, the brightness increases for several minutes (the flash phase), followed by a slow decay (the decay phase) which lasts up to an hour.  Formation of a flare is usually accompanied by a significant re-arrangement of the magnetic field configuration.

The maximum temperature in the core of a flare event can reach to 10 million Kelvin ! It causes bursts of radiation in gamma and x-ray, extreme ultraviolet, and microwaves. The physical process is the bremsstrahlung of electrons with energies 10-100 keV (an electron with 100 keV energy travels with 1/3 of speed of light).

Scientists classify solar flares according to their x-ray brightness in the wavelength range 1 to 8 Angstroms. There are 3 categories: X-class flares are big;  they are major events that can trigger planet-wide radio blackouts and long-lasting radiation storms. M-class flares are medium-sized; they can cause brief radio blackouts that affect Earth’s polar regions. Minor radiation storms sometimes follow an M-class flare. Compared to X- and M-class events, C-class flares are small with few noticeable consequences here on Earth. Large flares may be visible in white light as well !

Each category for x-ray flares has nine subdivisions ranging from, e.g., C1 to C9, M1 to M9, and X1 to X9. In this figure, the three indicated flares registered (from left to right) X2, M5, and X6. The X6 flare triggered a radiation storm around Earth nicknamed the Bastille Day event.


Class                  Peak (W/m2)  between 1 and 8 Angstroms


B                           I < 10-6


C                          10-6 < = I < 10-5


M                         10-5 < = I < 10-4


X                          I > = 10-4


The image below shows a large flare as recorded with EIT telescope of the SOHO spacecraft in previous solar cycle. We expect to see more solar flares in the coming 4-5 years.

X28 flare in EIT 195 -- The Sun unleashed a powerful flare on 4 November 2003 that could be the most powerful ever witnessed and probably as strong as anything detected since satellites were able to record these events n the mid-1970s. The still and video clip from the Extreme ultraviolet Imager in the 195A emission line captured the event. The two strongest flares on record, in 1989 and 2001, were rated at X20. This one was stronger scientists say. But because it saturated the X-ray detector aboard NOAA's GOES satellite that monitors the Sun, it is not possible to tell exactly how large it was. The consensus by scientists put it somewhere around X28.

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