Brhat Parasara Hora Sastra (Source of Mantras)

[venkat veeraraghavan]

Dear Vidvans,

In the attached file you will find the mantras for grahas taken from the Brihat Parasara Hora Sastra.

Most of these are in popular use except the ones for Chandra and Shukra.

In the attached 

इमं देवा असपत्नं .. is given as the mantra for Moon/Chandra

and अन्नात् परिश्रुतो रसं ...is given as the mantra for Venus/Shukra

What is the source of these mantras in Veda?

Also in popular practise these 2 mantras are substituted for 

आप्यायस्व समेतु ते विश्वतस्सोम​... (For Chandra)

and प्रवश्शुक्राय भानवे भरध्वम्... (For Shukra).

Is there a textual source for this different usage? 

If so why the difference?

Thanks,

Venkat