Upon FIRST seeing the new moon, one blesses while standing. Must be standing.
See Hilkhoth Brakhoth 10:19 (Temani Version):
If one didn't recite it the first night... he may do so until the 16th.
This is the mitzwaH to sanctify the new moon (ie: Kiddush haChodash).
Saying blessings on the Shabbath before Rosh Hodash (that mention the time for the new moon) is not called Kiddish haChodash. I believe the blessing on the Shabbath before are referred to as Birqath Hahodash.
Rosh Chodesh: http://www.biu.ac.il/JH/Parasha/eng/bo/levinger.html
According to Maimonides (Hilkhot Qiddush Ha-Hodesh - Sanctification of the New Moon, Chap. 1, halakha 3, 1170/78 C.E.) it becomes feasible (TO SEE THE MOON) "about a day after it (the Moon) adheres to the Sun" (=the true molad). But from his detailed calculations in chapter 17 (ibid), it becomes clear that 18 hours is the minimal time between the true molad and the first sighting of the Moon in the spring and 20 and one third hours in the autumn (Rabbi Raphael Ha-Lewi of Hanover, Tekhunat Ha-Shamayyim, section 91, which was copied into the common editions of the Code of Maimonides, at the end of Sefer Zemanim under the title: An Explanation of Maimonides' Words in Chapter 17 of the Treatise: Hilkhot Qiddush Ha-Hodesh).
Three Types of Molad
The True Molad = True Conjunction = the time at which the Moon (during its monthly orbital motion around the Earth) is between the Earth and the Sun, close to (or at the time of a solar eclipse: on) a straight line between the Earth and the Sun. At this time the Moon is generally not visible from the Earth. Since the Earth and the Moon move in elliptical orbits, not circles, the time interval between two consecutive true moladot varies. In the twentieth and the twenty-first centuries the length of the true lunar month is between approximately 29 days 6.5 hours and 29 days 20 hours (MEEUS, Astronomical Algorithms, 1991, p. 324).
The Mean Molad = Mean Conjunction = the average time, from which the true molad can deviate up to approximately 14 hours in either direction. The deviation possible varies, depending on the location of Earth in its path around the Sun. In Nissan the true molad can occur from approximately 6 hours before the mean molad until near to 14 hours after it, and in Tishri the true molad can occur from about 14 hours before the mean molad until around 6 hours after it (see Ibn Ezra, Commentary on the Tora, Ex. 12 2). The time interval between two mean moladot is currently (near the year 2000 C.E.): 29 days, 12 hours 44 minutes, 2.9 seconds [expressed in standard SI (Systeme International) seconds, measured by atomic clocks; MEEUS, 1991, p. 319]. This interval grows longer by approximately 0.1 SI seconds in 500 years. The great astronomer, Rabbi Raphael Ha-Lewi of Hanover calls this molad the "correct molad" (Luhot Ha-Ibbur, part 1, 1756, title page).
The Calendric (or: Common) Molad = Molad Ha-Ibbur = the Calendric (Mean) Conjunction calculated by the rules of the Hebrew calendar. It was close to the mean molad in the times of our Sages (1st cent.). Near the year 2000 C.E. it occurs about 2 hours after the mean molad (assuming that the common calendric molad, announced in synagogues on the Sabbath before Rosh Hodesh, refers to Jerusalem Mean Time i.e., the Israel Standard Time plus 21 minutes). The time interval between two consecutive calendric moladot is fixed by halakha at a constant 29 days, 12 hours, 44 minutes and one heleq (=1 part = 3.33 seconds).
Despite the first impression from the Talmudic text in Tractate Rosh Ha-Shana and the explanations of several of the commentators on it, we must adhere to the following facts:
a. An unaided sighting of the new (crescent) Moon is impossible before the passage of 15 to 20 hours or more after the true molad (depending on the season of the year). Sometimes sighting becomes possible in Tishri, in the evening after the occurrence of the mean molad, if it is not a late (zaqen) molad. (A late calendric molad, nowadays, still enables a sighting on the evening after it, e.g. in Tishri 5761 (28.9.2000), when observed from southwestern areas of Erez Yisrael!)
b. A late (zaqen) mean molad in Tishri, and even moreso in the other months, is a sufficient basis for confuting witnesses claiming to have seen the new (crescent) Moon on the evening after that mean molad. Thus only the second of the two dicta: "If it was born before midday" and "If it was not born before midday" is practically relevant.
c. It is possible to see the new (crescent) Moon at the beginning of the month only after the passage of at least 29 days from its previous first sighting, and only at a relatively low altitude above the western horizon, at around 20 minutes after sunset. In Tishri the Moon will be south of the W (west) point of the horizon.
d. It is possible to see the old (crescent) Moon at the end of the month, only if there remain at sunrise more than approximately 15 to 20 hours until the true molad. The old Moon can then appear only at a relatively low altitude above the eastern horizon around 20 minutes before sunrise. In Tishri the Moon will be found directly above the E (east) point of the horizon. (RaShY's explanations in our printed editions of Rosh Ha-Shana 20b, and also the opinion of the Baal Ha-Maor commenting on Rabbi Yizhaq Alfassi, ibid., seem to contradict the above, requiring therefore serious examination).
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Side Notes: 1) ALL LUNAR MONTHS ARE CALCULATED: There are 5 principles: ---First Molad of Tishray, Year 0, was at Day 2, 5 hours, 204/1080 ---Every 19 year cycle add 2 days, 16 hours, 595/1080 ---Every leap year adds 4 days, 8 hours, 876/1080 ---Every non leap year adds 5 days, 21 hours, 589/1080 ---Every month adds 29 days, 12 hours, 793/1080. So if I wanted the molad of Marcheshvan of the year 5700 = 19*300 I would ----------take 2 days 5 hours 204/1080 ----------add 300 x( 2 days, 16 hours, 595/1080) ----------add 29 days, 12 hours, 793/1080. (Source: Chapter 6, RMb"M, Laws of Lunar Months) It is possible to make a simple excel workbook exhibiting all 72000 months 2) The ANNOUNCED DAY OF THE MOLAD may differ from the Calculated day according to 4 exceptions presented in Chapter 7(For example Rosh Hashana can never occur on Sunday Wednesday or Friday). The RMb"M EXPLICITLY gives the reason for the laws in Chapter 7 as "because all calculations are averages"(paragraph 7) Over the past few years several people have offered programs doing this I am simply adding the point that it can be done in spreadsheet form for all known months of Jewish history