Table 1: Mean values of apparent Sun semi-diameter and errors (in arcsec) for the years 1997, 1998 and from January to June 1999. The Day column displays the number of observational days during the year, Obs brings the total number of observations, Diam - the mean semi-diameter, $\varepsilon$ - its error, $\sigma _{{\rm day}}$ - the error of mean daily value of semi-diameter and $\sigma _{1}$ - the error of a single observations
East transits West transits All transits

Year Day Obs Diam $\varepsilon$ $\sigma _{{\rm day}}$ $\sigma _{1}$ Day Obs Diam $\varepsilon$ $\sigma _{{\rm day}}$ $\sigma _{1}$ Day Obs Diam $\varepsilon$ $\sigma _{{\rm day}}$ $\sigma _{1}$

1997 144 1854 959.25 $\pm$ .03 .20 .60 69 802 959.32 $\pm$ .06 .22 .69 158 2656 959.27 $\pm$ .03 .20 .63

1998 135 1873 959.17 $\pm$ .02 .14 .46 140 2044 959.07 $\pm$ .02 .13 .44 162 3917 959.12 $\pm$ .02 .14 .45

1999 Jan.-June 70 1125 958.93 $\pm$ .03 .15 .53 73 1154 958.96 $\pm$ .02 .12 .42 84 2279 958.95 $\pm$ .02 .13 .47

All period 349 4852 959.15 $\pm$ .02 .17 .54 282 4000 959.10 $\pm$ .02 .15 .49 404 8852 959.13 $\pm$ .01 .16 .52

**Table 1:** Mean values of apparent Sun semi-diameter and errors (in arcsec) for the years 1997, 1998 and from January to June 1999. The *Day* column displays the number of observational days during the year, *Obs* brings the total number of observations, *Diam* - the mean semi-diameter, $\varepsilon$ - its error, $\sigma _{{\rm day}}$ - the error of mean daily value of semi-diameter and $\sigma _{1}$ - the error of a single observations
	East transits	West transits	All transits
Year	Day	Obs	Diam $\varepsilon$	$\sigma _{{\rm day}}$	$\sigma _{1}$	Day	Obs	Diam $\varepsilon$	$\sigma _{{\rm day}}$	$\sigma _{1}$	Day	Obs	Diam $\varepsilon$	$\sigma _{{\rm day}}$	$\sigma _{1}$
1997	144	1854	959.25 $\pm$ .03	.20	.60	69	802	959.32 $\pm$ .06	.22	.69	158	2656	959.27 $\pm$ .03	.20	.63
1998	135	1873	959.17 $\pm$ .02	.14	.46	140	2044	959.07 $\pm$ .02	.13	.44	162	3917	959.12 $\pm$ .02	.14	.45
1999 Jan.-June	70	1125	958.93 $\pm$ .03	.15	.53	73	1154	958.96 $\pm$ .02	.12	.42	84	2279	958.95 $\pm$ .02	.13	.47
All period	349	4852	959.15 $\pm$ .02	.17	.54	282	4000	959.10 $\pm$ .02	.15	.49	404	8852	959.13 $\pm$ .01	.16	.52

Table 1 gives annual statistics of the observations for the whole observational period. Notice that the errors of the mean daily value of the semi-diameter $\sigma _{{\rm day}}$ and the error of single observations $\sigma _{1}$ , became smaller in 1998 and 1999, than in 1997. This was due to a narrower observation bandpass and to improvements in the observational routine.

Normally, about 20 or more (up to 30) observations, on each side of the Sun's meridian cross are made in summer months. These figures fall to about 10 or less, on each side, in June when the Sun is comparatively low above the horizon. The Summer months, from December to February, are mostly rainy, resulting in less effective observing time than in the Winter, from June to August, but many more diameter measurements.

For the one-year period here considered there are 162 days of observation. In spite of the hard weather conditions of the observations, the astrolabe performed rather stably. No dependence was found of the observed semi-diameter on time length of observations (that vary from 2 to 7 minutes) and azimuth.

Table 2: Monthly average apparent semi-diameters (in arcsec) from July 1998 to June 1999. The Day column displays the number of observational days during the month, the Obs column brings the total number of observations. Columns Diam and $\varepsilon$ bring the mean semi-diameter and rms error
East transits West transits All transits

Month Day Obs Diam $\varepsilon$ Day Obs Diam $\varepsilon$ Day Obs Diam $\varepsilon$

July 1998 14 127 959.33 $\pm$ 0.10 16 187 959.12 $\pm$ 0.04 17 314 959.22 $\pm$ 0.05

August 13 194 959.15 $\pm$ 0.09 13 217 959.08 $\pm$ 0.04 14 411 959.11 $\pm$ 0.05

September 9 126 959.12 $\pm$ 0.05 12 212 959.08 $\pm$ 0.05 13 338 959.10 $\pm$ 0.03

October 6 82 959.13 $\pm$ 0.08 8 145 959.12 $\pm$ 0.10 9 227 959.12 $\pm$ 0.07

November 8 159 959.28 $\pm$ 0.06 10 184 959.14 $\pm$ 0.06 11 343 959.20 $\pm$ 0.04

December 11 175 959.45 $\pm$ 0.06 12 189 959.34 $\pm$ 0.05 14 364 959.39 $\pm$ 0.03

January 1999 13 224 959.06 $\pm$ 0.08 13 224 959.16 $\pm$ 0.04 15 448 959.11 $\pm$ 0.04

February 10 219 958.78 $\pm$ 0.04 12 189 958.85 $\pm$ 0.05 13 408 958.82 $\pm$ 0.03

March 14 257 958.88 $\pm$ 0.06 12 217 958.94 $\pm$ 0.03 16 474 958.91 $\pm$ 0.04

April 14 217 958.88 $\pm$ 0.06 13 237 958.83 $\pm$ 0.06 15 454 958.86 $\pm$ 0.04

May 9 116 958.07 $\pm$ 0.13 13 181 959.02 $\pm$ 0.05 14 297 959.00 $\pm$ 0.05

June 10 92 959.04 $\pm$ 0.10 10 107 958.94 $\pm$ 0.06 11 199 958.99 $\pm$ 0.06

All period 131 1988 959.08 $\pm$ 0.03 144 2288 959.05 $\pm$ 0.02 162 4276 959.07 $\pm$ 0.02

**Table 2:** Monthly average apparent semi-diameters (in arcsec) from July 1998 to June 1999. The *Day* column displays the number of observational days during the month, the *Obs* column brings the total number of observations. Columns *Diam* and $\varepsilon$ bring the mean semi-diameter and rms error
	East transits	West transits	All transits
Month	Day	Obs	Diam $\varepsilon$	Day	Obs	Diam $\varepsilon$	Day	Obs	Diam $\varepsilon$
July 1998	14	127	959.33 $\pm$ 0.10	16	187	959.12 $\pm$ 0.04	17	314	959.22 $\pm$ 0.05
August	13	194	959.15 $\pm$ 0.09	13	217	959.08 $\pm$ 0.04	14	411	959.11 $\pm$ 0.05
September	9	126	959.12 $\pm$ 0.05	12	212	959.08 $\pm$ 0.05	13	338	959.10 $\pm$ 0.03
October	6	82	959.13 $\pm$ 0.08	8	145	959.12 $\pm$ 0.10	9	227	959.12 $\pm$ 0.07
November	8	159	959.28 $\pm$ 0.06	10	184	959.14 $\pm$ 0.06	11	343	959.20 $\pm$ 0.04
December	11	175	959.45 $\pm$ 0.06	12	189	959.34 $\pm$ 0.05	14	364	959.39 $\pm$ 0.03
January 1999	13	224	959.06 $\pm$ 0.08	13	224	959.16 $\pm$ 0.04	15	448	959.11 $\pm$ 0.04
February	10	219	958.78 $\pm$ 0.04	12	189	958.85 $\pm$ 0.05	13	408	958.82 $\pm$ 0.03
March	14	257	958.88 $\pm$ 0.06	12	217	958.94 $\pm$ 0.03	16	474	958.91 $\pm$ 0.04
April	14	217	958.88 $\pm$ 0.06	13	237	958.83 $\pm$ 0.06	15	454	958.86 $\pm$ 0.04
May	9	116	958.07 $\pm$ 0.13	13	181	959.02 $\pm$ 0.05	14	297	959.00 $\pm$ 0.05
June	10	92	959.04 $\pm$ 0.10	10	107	958.94 $\pm$ 0.06	11	199	958.99 $\pm$ 0.06
All period	131	1988	959.08 $\pm$ 0.03	144	2288	959.05 $\pm$ 0.02	162	4276	959.07 $\pm$ 0.02

An analysis of the apparent semi-diameter and some studies concerning the variation of the astroclimatic conditions at ON are in preparation. Here instead we focus on the monthly average values, whose distribution is approximately normal.

In Table 2 are presented the number of the days of observation, the total number of observations during a month and the monthly averages. The results of the East and West observations are tabulated as well as an average of all transits.

The error of a single measurement of the solar semi-diameter for the period of 1998-1999 is 0 $\hbox{$.\!\!^{\prime\prime}$ }$ 51 for the East and 0 $\hbox{$.\!\!^{\prime\prime}$ }$ 43 for West observations.

Table 1 indicates a decreasing trend of 0 $\hbox{$.\!\!^{\prime\prime}$ }$ 16 per year for the apparent semi-diameter. The average value for 1998.25 is 959 $\hbox{$.\!\!^{\prime\prime}$ }$ 13. This represents a difference of 0 $\hbox{$.\!\!^{\prime\prime}$ }$ 4 for the average value of Calern observations for the period 1975 - 1998 (Laclare et al. 1999a, 1999b). However, similar trends and differences can also be found within the Calern series. On the other hand, discrepancies in the average values can be expected due to differences in the effective observational wavelength, in the operating definition of the solar border and in the prevailing local atmospheric conditions.

Complete results of the daily series are updated every month and can be retrieved in electronic form from the SIMBAD database and from the homepage of ON (http://obsn.on.br/radius/). For the electronic form, the daily data are given: the number of observations, the semi-diameter mean value, its rms error, the date in format YYMMDD, the Modified Julian Date and the error of a single observation.

2 Results and discussion